Convert API refs to StackBox from ActivityStack.

- Removed IActivityManager.getStacks() since getStackBoxes() is better.
- Made createStacks operate relative to StackBox instead of TaskStack.
- Made resizeStack into resizeStackBox.

Change-Id: I7a0e1f4e34f399b4fd1180c60cc3989f9c2433f3
7 files changed