Ensuring that isPictureInPictureMode() mirrors dispatched mode.

- When transitioning out of picture-in-picture, we schedule
  onPictureInPictureModeChanged() when the animation starts, so the check
  in isInPictureInPicture() should match this as well.

Bug: 36845078
Test: Updated android.server.cts.ActivityManagerPinnedStackTests to also
      whenever PipActivity PiP mode changes, that it matches the activity
      state as well.

Change-Id: I31d7a3307b01b818e867f86a4553a363c7f11307