[flexiglass] Add an InputMethod module + IME switcher button to bouncer.

* Bonus: Add a new utility extension function `subscribersIncreased` to
  `Flow`, and remove the unused `stateFlow` function.

Fix: 300459199
Test: Manually tested by adding another input language and successfully
 switching it in the password bouncer using the button.
Test: Added unit tests, existing ones still pass.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: Id223990bfac777d5dcad071d6428824e94801ca0