Creating PinnedStackController.

- Creating a PinnedStackController to keep track of the state of the PIP
  to prevent changes in the system (ie. IME showing) and user interaction
  from clobbering each other.
- Refactoring calls in AM into WM/controller

Test: android.server.cts.ActivityManagerPinnedStackTests

Change-Id: Ie59dfd45d5c54764ba69a589b3b8148845e92cc3
Signed-off-by: Winson Chung <winsonc@google.com>