[camerax] ☂️ Clean up Dart Tests

[camsim99]

As the android_camera_camerax plugin has been under development, there have a number of tasks that have not blocked PRs but are important technical debt to pay down. This umbrella issue covers those tasks that apply to cleaning up tests in this plugin:

• [ ] Consider implementing a setUp() method for the plugin Dart test since several of the same setup steps are involved.
• [x] #138633
• [ ] Condense test mock files that contain same mocks, namely mocks for TestInstanceManagerHostApi.
• [ ] Reduce assertion calls in tests that loop over various types (example)
• [ ] Add test for behavior of using weak references of plugin implementation in callback methods to avoid memory leaks (see discussion).
• [ ] Reduce parameters that are not necessary to be visible for testing (now that I've added a proxy, some of these can be private).

[camsim99]

• [ ] Consider refactoring the FakeAndroidCameraCameraX classed used in the plugin Dart test for testing, since it is likely these objects used in the test can be mocked without using it.

Concerning this point, this may become easier for future wrappings with https://github.com/flutter/flutter/issues/134777, but I think a good starting point would be to follow WebView and use a proxy class like https://github.com/flutter/packages/blob/main/packages/webview_flutter/webview_flutter_android/lib/src/android_proxy.dart#L16 (which can be used to created detached or mock objects) that is passed to the WebViewController (https://github.com/flutter/packages/blob/main/packages/webview_flutter/webview_flutter_android/lib/src/android_webview_controller.dart#L31).

cc @gmackall as this is a follow up to https://github.com/flutter/packages/pull/5287#issuecomment-1789271416