Pair ActivityStacks with Displays

- Introduce concept of ActivityStacks residing on Displays and able
to be decoupled and moved around.
- Add a new interface, IActivityContainer for clients to handle
ActivityStacks.
- Abandon ordering of stacks based on mStackState and instead use
ActivityDisplayInfo.stacks<ActivityStack> ordering.

Progress towards closing bug 12078972.

Change-Id: I7785b61c26dc17f432a4803eebee07c7415fcc1f