28884 Make FlutterEngineGroup support initialRoute (platform-ios, platform-android, cla: yes)

28886 Bump maximum supported sdk to 30 for Robolectric (platform-android, cla: yes, waiting for tree to go green)

28891 Avoid GCing aggressively during startup, and forward trim messages beyond low to application (platform-android, cla: yes)

28894 Destroy overlay surfaces when the rasterizer is torn down (platform-ios, platform-android, cla: yes, waiting for tree to go green, platform-fuchsia)

28903 Use the systrace recorder if systracing is enabled at startup, and enable systracing in release mode on Android (platform-android, cla: yes, waiting for tree to go green, needs tests)

28921 Deprecate SplashScreenProvider (platform-android, cla: yes, needs tests)