Don't allow any app to specify rotation unless in home task

The previous logic was racy/buggy in a sense so that the non-home app
could still force the orientation when the divider was still minimized.
The logic is cleaned up so only the home task can force the orientation
in multi-window.

Change-Id: I9eccf86d9fa442375d6eca400cb6ac0815c9fa10
Fixes: 28675152