[webview_flutter_android] Updates Dart and Java InstanceManagers

[bparrishMines]

These changes fix a potential bug caused by a hot restart that would fail to call FlutterApi.create for a static native Object.

Updates to Java InstanceManager:

• Added the clear method. This method was added to be able to clear the InstanceManager in the event of a hot restart.
• Since the InstanceManager can now be cleared on a hot restart, it now requires that instances added with addHostCreatedInstance to be unique. And identifiers used in addDartCreatedInstance must be unique.

Updates to Dart InstanceManager:

• The initial call to JavaObject.globalInstanceManager makes a pigeon call to clear the native InstanceManager.
• Adds the @immutable annotation to Copyable.

This also raises the pigeon version

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.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[stuartmorgan-g]

LGTM with one suggestion

[stuartmorgan-g]

The less error-prone way of doing lazy initialization is:

static InstanceManager globalInstanceManager = _initInstanceManager();

static final InstanceManager _initInstanceManager() {
  WidgetsFlutterBinding.ensureInitialized();
  // Clears the native `InstanceManager` on initial use of the Dart one.
  InstanceManagerHostApi().clear();
  return InstanceManager(
    onWeakReferenceRemoved: (int identifier) {
      JavaObjectHostApiImpl().dispose(identifier);
    },
  );
}

(For non-statics, you can do the same thing but with late final to make it lazy.)

[bparrishMines]

cc @camsim99 FYI when updating the InstanceManager.

[bparrishMines]

Updated example

/// Global instance of [InstanceManager].
static final InstanceManager globalInstanceManager = _initInstanceManager();

static InstanceManager _initInstanceManager() {
  WidgetsFlutterBinding.ensureInitialized();
  // Clears the native `InstanceManager` on initial use of the Dart one.
  InstanceManagerHostApi().clear();
  return InstanceManager(
    onWeakReferenceRemoved: (int identifier) {
      JavaObjectHostApiImpl().dispose(identifier);
     },
  );
}

[stuartmorgan-g]

Oops, I'll fix my copy to avoid confusion. Sorry about that.