Cache reference to stack objects we use a lot in AM.

This helps with fast access since we are no longer using stack ids
to look-up stack objects.
Also,
- Made ActivityDisplay.mStacks private so no one out the ActivityDisplay
class changing the stack list order.
- Changed some methods to pass stack objects vs. ids.

Test: Existing tests pass.
Test: go/wm-smoke
Change-Id: I04490cd505c4251e115043fa2ba5134d9a641846