Update Android WebView to handle garbage collection automatically

Action Items:

- [x] Add `Copyable` class from iOS implementation.
- [x] Update Dart `InstanceManager` with `Finalizer` support.
- [x] Update Java `InstanceManager` with support for tracking garbage collected objects.
- [x] Add dispose pigeon methods for `HostApi` and `FlutterApi`.
- [x] Replace callback methods with anonymous functions.