Remove Linux orchestrator builds that only kick off one other build and wait (flutter#55186)

"Orchestrator" builders are top-level builds that can perform some caching, kick off "drone" sub-builds, and then do things with those build artifacts like run tests, or "generators" that do work on those artifacts.  See more details in https://flutter.dev/go/engine-build-definition-language.

Some orchestrators in [`ci/builders`](https://github.com/flutter/engine/tree/38e37ce9879b2efe4890a8d09f4207dc5686a1bd/ci/builders) _only_ kick off one sub-build.  This is suboptimal because:
1. The orchestrator uses a bot that does nothing except essentially idle.
2. The orchestrator has some overhead, so the orchestrator + single drone sub-build takes longer than just the sub-build run alone.
3. The "drone" sub-build often has a generic name like "Linux Engine Drone" which makes it difficult to keep track of which build is which.

This PR:
1. Hoists a few of these Linux builds into [`ci/builders/standalone`](https://github.com/flutter/engine/tree/38e37ce9879b2efe4890a8d09f4207dc5686a1bd/ci/builders/standalone) which are run as individual builds, and removes the orchestrator.  
2. Swaps the recipe from `engine_v2/engine_v2` (orchestrator) to `engine_v2/builder` (standalone).  
3. Moves the `drone_dimensions` from the orchestrator build json into .ci.yaml.

| Builder    | Before | After |
| -------- | ------- | ------- |
| `Linux clangd`  | [9m & 2 Linux bots](https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20clangd/6272/overview)  ([drone build](https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20Engine%20Drone/2689993/overview))  | [6m & 1 Linux bot](https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20clangd/6292/overview) |
| `Linux linux_android_emulator_skia_tests_34`  | [29m & 2 Linux bots](https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20linux_android_emulator_skia_tests_34/663/overview) ([drone build](https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20Engine%20Drone/2690008/overview))  | [25m & 1 Linux bot](https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20linux_android_emulator_skia_tests_34/678/overview) |

Part of flutter#155041

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

Author: jmagman

.ci.yaml

@@ -98,9 +98,10 @@ targets:
     bringup: true
     enabled_branches:
       - main
-    recipe: engine_v2/engine_v2
+    recipe: engine_v2/builder
     properties:
       config_name: linux_android_emulator_skia
+      kvm: "1"
       dependencies: >-
         [
           {"dependency": "goldctl", "version": "git_revision:720a542f6fe4f92922c3b8f0fdcc4d2ac6bb83cd"}
@@ -118,9 +119,10 @@ targets:
   - name: Linux linux_android_emulator_skia_tests_34
     enabled_branches:
       - main
-    recipe: engine_v2/engine_v2
+    recipe: engine_v2/builder
     properties:
       config_name: linux_android_emulator_skia_34
+      kvm: "1"
       dependencies: >-
         [
           {"dependency": "goldctl", "version": "git_revision:720a542f6fe4f92922c3b8f0fdcc4d2ac6bb83cd"}
@@ -382,7 +384,7 @@ targets:
       - flutter_frontend_server/**
 
   - name: Linux clangd
-    recipe: engine_v2/engine_v2
+    recipe: engine_v2/builder
     properties:
       config_name: linux_unopt_debug_no_rbe
 

ci/builders/linux_android_emulator_skia.json

@@ -0,0 +1,81 @@
+{
+    "gclient_variables": {
+        "use_rbe": true
+    },
+    "gn": [
+        "--android",
+        "--android-cpu=x64",
+        "--no-lto",
+        "--rbe",
+        "--no-goma",
+        "--target-dir",
+        "ci/android_emulator_skia_debug_x64"
+    ],
+    "dependencies": [
+        {
+            "dependency": "goldctl",
+            "version": "git_revision:720a542f6fe4f92922c3b8f0fdcc4d2ac6bb83cd"
+        }
+    ],
+    "name": "ci/android_emulator_skia_debug_x64",
+    "description": "Build for debug mode x64 Android Skia scenario app tests.",
+    "ninja": {
+        "config": "ci/android_emulator_skia_debug_x64",
+        "targets": [
+            "flutter/impeller/toolkit/android:unittests",
+            "flutter/shell/platform/android:flutter_shell_native_unittests",
+            "flutter/testing/scenario_app"
+        ]
+    },
+    "tests": [
+        {
+            "language": "dart",
+            "name": "Android Scenario App Integration Tests (Skia)",
+            "test_timeout_secs": 900,
+            "max_attempts": 2,
+            "test_dependencies": [
+                {
+                    "dependency": "android_virtual_device",
+                    "version": "android_35_google_apis_x64.textpb"
+                },
+                {
+                    "dependency": "avd_cipd_version",
+                    "version": "build_id:8740267484269553649"
+                }
+            ],
+            "contexts": [
+                "android_virtual_device"
+            ],
+            "script": "flutter/testing/scenario_app/bin/run_android_tests.dart",
+            "parameters": [
+                "--out-dir=../out/ci/android_emulator_skia_debug_x64",
+                "--no-enable-impeller"
+            ]
+        },
+        {
+            "language": "dart",
+            "name": "Android Scenario App Integration Tests (Skia, SurfaceTexture)",
+            "test_timeout_secs": 900,
+            "max_attempts": 2,
+            "test_dependencies": [
+                {
+                    "dependency": "android_virtual_device",
+                    "version": "android_35_google_apis_x64.textpb"
+                },
+                {
+                    "dependency": "avd_cipd_version",
+                    "version": "build_id:8740267484269553649"
+                }
+            ],
+            "contexts": [
+                "android_virtual_device"
+            ],
+            "script": "flutter/testing/scenario_app/bin/run_android_tests.dart",
+            "parameters": [
+                "--out-dir=../out/ci/android_emulator_skia_debug_x64",
+                "--no-enable-impeller",
+                "--force-surface-producer-surface-texture"
+            ]
+        }
+    ]
+}