Renamed some members of AppWindowToken (49/n)

- Renamed fillParent to occludesParent since it indicates the
activity is opaque and fills the entire space of this task.
- Renamed canTurnScreenOn to currentLaunchCanTurnScreenOn since
it only applies to the current launch.
Also, made additional clean-ups relating to the rename.

Bug: 80414790
Test: Existing tests pass
Change-Id: I212fae928d9de87e8324d9702eb96661888f3b46