Updating moveTaskToBack to handle PiP activities

- When a PiP activity calls moveTaskToBack(), we should fall through to
  the same behaviour as dismissing the PiP if there are no other tasks
  to show.

Bug: 36034065
Test: android.server.cts.ActivityManagerPinnedStackTests
Test: #testMovePipToBackWithNoFullscreenStack
Test: #testMovePipToBackWithVisibleFullscreenStack
Test: #testMovePipToBackWithHiddenFullscreenStack

Change-Id: I83c3d27dabc031546f9f1cf20b808bb8f13d4646
Signed-off-by: Winson Chung <winsonc@google.com>