Changing initial PIP bounds specifications.

- Change from a absolute bounds to a size, gravity and inset to
  allow easier tweaking of initial pinned stack bounds.

Test: android.server.cts.ActivityManagerPinnedStackTests
Test: #testPinnedStackDefaultBounds
Test: #testPinnedStackMovementBounds

Change-Id: I82a102bbd2d83259c6c84915071d1a16728e3aa7