[various] Fixes unawaited_futures violations

[stuartmorgan-g]

This option had been disabled to match flutter/flutter, but the reason it was disabled there was "too many false positives", mostly around animation. That doesn't apply to most packages here, and we've had a number of production bugs—especially in plugins, that use async heavily in ways that are intended to be client-awaitable—that this would have caught.

This PR:

• Enables the option at the repo level.
• Permanently (unless the owners decide to change it) opts out animations and go_router, both of which looked like mostly or entirely false positives.
• Temporarily opted out a few plugins that have a lot of violations that should be handled in their own PRs later (camera_android_camerax, most of webview_flutter).
• Fixes all remaining violations.

In many cases this PR is behavior-changing, replacing implicitly unawaited futures that did not seem obviously intentional with awaited futures, so non-test code in particular should be reviewed carefully to make sure the changes are correct. All of the changes are manual, not fix-generated.

Part of flutter/flutter#127323

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[chunhtai]

LGTM for go_router and go_router_builder

[reidbaker]

Unrelated to this pr. I wish we used absolute paths instead of relative paths when the absolute path has more meaning.

I know that effective dart says to prefer relative paths but in this case it matters more that we are using the root analysis_options.yaml than the one 3 folders up.
https://dart.dev/effective-dart/usage#prefer-relative-import-paths

[stuartmorgan-g]

I agree that would be better in this case, but I don't think it's actually possible; there's no package at the root.

I guess we could make a local package with the shared options, and then depend on that in every single pubspec.yaml so we could use a package: URL, but that would be pretty ugly. And the local package reference in the pubspec would have to be... a relative path.

[reidbaker]

Mild preference for ignore for file in the test than opting out the directory.

[tarrinneal]

pigeon and shared_prefs are fine, assuming ci doesn't hang on pigeon tests

[ditman]

Not sure why I left some of the calls sneakily unawaited, but CI will tell. 🚀

[stuartmorgan-g]

• All existing and new tests are passing.

Looks like this was aspirational. Not too many failures though; I'll take a look tomorrow.

[stuartmorgan-g]

This change was incorrect; the test never simulates running the callback, causing the test to hang; I'll make it unawaited with a comment.

[stuartmorgan-g]

Apparently the fact that the error wasn't being caught by the returned future was (or at least has become) intentional behavior, since there was a unit test that this change broke. I doubt very much that having _initialization not be guaranteed to be null when the future completes is intentional though, so I changed it to return the catchError wrapper, but made the wrapper rethrow after nulling _initialization.