[flexiglass] Structured concurrency fixes

- Breaks up BaseActivatable to Hydrator and ExclusiveActivatable
- Hydrator managed a set of static children
- SysUiViewModel no longer implements Activatable
- All converted view-models properly migrated to implement SysUiViModel
  and extend ExclusiveActivatable if they need to be Activatable
- The existing usage of hydratedStateOf migrated to use a Hydrator
- rememberViewModel and viewModel view-model factory functons changes to
  only activate view-models that are activatable

Bug: 354270224
Test: unit tests refactored and moved around to follow the new class
split
Test: manually verified no harm done with a Flexiglass smoke test: shade
and QS over lockscreen, bouncer, unlock, shade and QS over unlocked
device, relock
Flag: com.android.systemui.scene_container

Change-Id: I138795d49271923c14d677a64b1488c72d816282