[google_maps_flutter] Partial Android host API Pigeon conversion (flutter#6967)

Converts the host API methods other than the various map-object-update methods to Pigeon, as a starting point for a Pigeon conversion. This introduces the Pigeon structure for the main APIs, without getting into complex object serialization. Future work includes:
- Converting the update methods.
- Converting the calls from native to Dart (Flutter API in Pigeon terms).

Also replaces the very minimal Dart unit tests with full tests of all the new methods.

Part of flutter#117907

Author: stuartmorgan-g

packages/google_maps_flutter/google_maps_flutter_android/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 2.10.0
+
+* Converts some platform calls to Pigeon.
+
 ## 2.9.1
 
 * Converts inspector interface platform calls to Pigeon.

packages/google_maps_flutter/google_maps_flutter_android/android/src/main/java/io/flutter/plugins/googlemaps/Convert.java

@@ -439,11 +439,15 @@ static Object latLngToJson(LatLng latLng) {
 
   static Messages.PlatformLatLng latLngToPigeon(LatLng latLng) {
     return new Messages.PlatformLatLng.Builder()
-        .setLat(latLng.latitude)
-        .setLng(latLng.longitude)
+        .setLatitude(latLng.latitude)
+        .setLongitude(latLng.longitude)
         .build();
   }
 
+  static LatLng latLngFromPigeon(Messages.PlatformLatLng latLng) {
+    return new LatLng(latLng.getLatitude(), latLng.getLongitude());
+  }
+
   static Object clusterToJson(String clusterManagerId, Cluster<MarkerBuilder> cluster) {
     int clusterSize = cluster.getSize();
     LatLngBounds.Builder latLngBoundsBuilder = LatLngBounds.builder();
@@ -500,17 +504,12 @@ static LatLng toLatLng(Object o) {
     return new LatLng(toDouble(data.get(0)), toDouble(data.get(1)));
   }
 
-  static Point toPoint(Object o) {
-    Object x = toMap(o).get("x");
-    Object y = toMap(o).get("y");
-    return new Point((int) x, (int) y);
+  static Point pointFromPigeon(Messages.PlatformPoint point) {
+    return new Point(point.getX().intValue(), point.getY().intValue());
   }
 
-  static Map<String, Integer> pointToJson(Point point) {
-    final Map<String, Integer> data = new HashMap<>(2);
-    data.put("x", point.x);
-    data.put("y", point.y);
-    return data;
+  static Messages.PlatformPoint pointToPigeon(Point point) {
+    return new Messages.PlatformPoint.Builder().setX((long) point.x).setY((long) point.y).build();
   }
 
   private static LatLngBounds toLatLngBounds(Object o) {