Fixing some issues with the assistant stack

- Only boost window layers for the docked stack above the windows for
  stacks that are split with the docked stack. In addition, only adjust
  the assistant windows if the docked stack windows are boosted.
- In addition, mark activities in the fullscreen stack as invisible when
  they are under another task that returns to another stack. This prevents
  all the fullscreen tasks from being visible when a translucent activity
  is started from the assistant stack.
- Also fix an issue where returning from the assistant after launching a
  fullscreen task would incorrectly being the fullscreen stack forward
  (since the assistant stack is finishing and considered translucent).
  In such cases, use the return-type of the assistant stack task to bring
  the home stack forward instead.

Bug: 37527727
Test: android.server.cts.ActivityManagerAssistantStackTests
Test: go/wm-smoke

Change-Id: Ie81fbebe016f4854c7edebf382c0c3255f1b6471
Signed-off-by: Winson Chung <winsonc@google.com>