[flexiglass] Password bouncer beahvior fixes.

1. Keyboard is reshown when returning to the password bouncer after
   throttling
2. Dismissing the keyboard still exits the bouncer scene and returns to
   the lockscreen scene
3. Keyboard is automatically reshown after the throttling countdown
   finishes, if the user stayed on the bouncer scene
4. Dismissing the throttling dialog doesn't exit the bouncer scene

Fix: 306520416
Test: see above for manual test descriptions
Test: test-driven development (TDD): new unit tests added, written before changes, then changes applied
until new unit tests all passed
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: Ia908a5eced47d59eab2633257371dbfc04b46478