da447b9297
Fixes internal bug b/303652511 and includes a regression test. The TL;DR of the bug is that when re-launching a singleton-cached-engine Flutter activity using FLAG_ACTIVITY_CLEAR_TASK, the teardown of the previous Flutter activity interleaves with the creation of the new Flutter activity, resulting in the Flutter engine ending up incorrectly in the AppLifecycleState.detached state. This then results in the app being completely unresponsive because Flutter doesn't draw frames in this state. I don't know if the issue is reproducible in production under normal user operation, but I'm concerned it very easily could be. There's also nothing stopping other apps or Android system code from launching an app using this flag.