[flexiglass] Centralizes automatic scene changing business logic.

We had automatic scene changing business logic spread out amongst
various scene-specific interactors. This was suboptimal for a few reasons:
1. Hard to reason about what changes the scenes and when, when it's all
   distributed
2. Hard to gate this logic based on the feature flag; causing memory and
   boot time regressions as seen in the attached files, because more
   resources were being used at startup even when the feature was off

This CL moves business logic from LockScreenSceneInteractor and
BouncerInteractor into SystemUiDefaultSceneContainerStartable and makes
sure it's all gated behind the feature flag.

Fix: 280883900, 289353099, 289726647
Test: new unit tests added
Test: manually verified in system UI that locking the device moves back
to the Lockscreen scene and unlocking the device moves to the Gone scene

Change-Id: I1dc46fc2d1d71f1b9baddc822ef69f5ea7b21cfc