Add scene plugin lifecycle events (flutter#175866)

This PR forwards scene lifecycle events from the SceneDelegate to
registered plugins.

This PR introduced 3 new public APIs:
* `FlutterSceneLifeCycleDelegate` (protocol) - This is the protocol that
plugins conform to
* `FlutterSceneLifeCycleProvider` (protocol) - This is the protocol that
a SceneDelegate can conform to instead of `FlutterSceneDelegate`
* `FlutterPluginSceneLifeCycleDelegate` (interface) - This is the class
object that a SceneDelegate using `FlutterSceneLifeCycleProvider` can
use to forward scene events to

The flow of events are as followed when SceneDelegate subclasses the
`FlutterSceneDelegate`:
* `FlutterSceneDelegate` receives `sceneDidBecomeActive` event from
UIKit
* `FlutterSceneDelegate` forwards `sceneDidBecomeActive` to
`FlutterPluginSceneLifeCycleDelegate`
* `FlutterPluginSceneLifeCycleDelegate` loops through each
`FlutterEngine` it has and forwards `sceneDidBecomeActive` to the
engine's `FlutterEnginePluginSceneLifeCycleDelegate`
* `FlutterEnginePluginSceneLifeCycleDelegate` loops through each plugin
that conforms to `FlutterSceneLifeCycleDelegate` it has and forwards
`sceneDidBecomeActive` to the plugin
* `FlutterPluginSceneLifeCycleDelegate` forwards `sceneDidBecomeActive`
to `FlutterPluginAppLifeCycleDelegate` using
`sceneDidBecomeActiveFallback`
* `FlutterPluginAppLifeCycleDelegate` loops through all plugins and
calls `applicationDidBecomeActive` if the plugin does not conform to
`FlutterSceneLifeCycleDelegate`
  
When using `FlutterSceneLifeCycleProvider` instead of
`FlutterSceneDelegate`, scene events are manually forwarded to
`FlutterPluginSceneLifeCycleDelegate` by the iOS developer. For example:

```swift
class SceneDelegate: UIResponder, UIWindowSceneDelegate, FlutterSceneLifeCycleProvider {

  var sceneLifeCycleDelegate: FlutterPluginSceneLifeCycleDelegate = FlutterPluginSceneLifeCycleDelegate()

  func sceneDidBecomeActive(_ scene: UIScene) {
    sceneLifeCycleDelegate.sceneDidBecomeActive(scene)
  }
```

Fixes flutter#174398.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

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

**Note**: The Flutter team is currently trialing the use of [Gemini Code
Assist for
GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code).
Comments from the `gemini-code-assist` bot should not be taken as
authoritative feedback from the Flutter team. If you find its comments
useful you can update your code accordingly, but if you are unsure or
disagree with the feedback, please feel free to wait for a Flutter team
member's review for guidance on which automated comments should be
addressed.

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: vashworth

.ci.yaml

@@ -4306,6 +4306,26 @@ targets:
       - engine/**
       - DEPS
 
+  - name: Mac module_uiscene_test_ios
+    recipe: devicelab/devicelab_drone
+    bringup: true
+    timeout: 60
+    properties:
+      dependencies: >-
+        [
+          {"dependency": "ruby", "version": "ruby_3.1-pod_1.13"}
+        ]
+      tags: >
+        ["devicelab", "hostonly", "mac"]
+      task_name: module_uiscene_test_ios
+    runIf:
+      - dev/**
+      - packages/flutter_tools/**
+      - bin/**
+      - .ci.yaml
+      - engine/**
+      - DEPS
+
   - name: Mac_benchmark platform_view_macos__start_up
     presubmit: false
     recipe: devicelab/devicelab_drone

TESTOWNERS

@@ -275,6 +275,7 @@
 /dev/devicelab/bin/tasks/build_android_host_app_with_module_aar.dart @mboetger @flutter/android
 /dev/devicelab/bin/tasks/build_android_host_app_with_module_source.dart @mboetger @flutter/android
 /dev/devicelab/bin/tasks/module_test_ios.dart @vashworth @flutter/tool
+/dev/devicelab/bin/tasks/module_uiscene_test_ios.dart @vashworth @flutter/ios
 /dev/devicelab/bin/tasks/native_assets_ios_simulator.dart @dcharkes @flutter/ios
 /dev/devicelab/bin/tasks/native_ui_tests_macos.dart @cbracken @flutter/desktop
 /dev/devicelab/bin/tasks/platform_channel_sample_test_macos.dart @cbracken @flutter/desktop