Updating picture-in-picture API.

- Consolidating to enterPictureInPictureMode(), the new method will
  attempt to put the activity into picture-in-picture mode if the
  activity is visible or pausing in a state that would allow us to
  pip it.  Also consolidate the setting of the PiP aspect ratio and
  actions into setPictureInPictureArgs().
- Fixing issue with onPause not completing when moving the
  paused activity between stacks while dispatching onPause

Bug: 33692987
Test: android.server.cts.ActivityManagerPinnedStackTests

Change-Id: I3af2365f31a9b95de4a92eae46b77108947b2a49