Enable window-mode propagation to tasks.

Removes a number of places where both bounds and windowingMode were
hard-coded.

ActivityDisplay:
- Added support for unspecified windowing mode (removing hard-coded
  FULLSCREEN).
- Enabled propagation of windowing-mode from display to stacks/tasks
  (via unspecified mode).

ActivityStack:
- Don't always set override-bounds if UNDEFINED. Instead, rely on
  inheritance from ActivityDisplay. Continue to validate defined
  modes though.

ActivityStackSupervisor:
- Since task to stack is 1-to-1 (except potentially in split modes),
  have isValidLaunchStack return false most of the time.

TaskRecord:
- Fixed a NPE where override-bounds wasn't being checked for nullness
  when recording lastNonFullscreenBounds.

Change-Id: Ic67419464e5c7863deb31ba4a79eaa60d4f47df0
Bug: 71028874
Test: go/wm-smoke