- It's sent from PipTransitionController when we start and finish the
  animation
- Removed the restriction that onPictureInPictureUiStateChanged can only
  be sent to a pinned task, since this entering PiP callback would be
  received when the task is still in full-screen

Note that there is no isExitingPip API being added here since
- apps normally do not restore overlays such as video controls when
  exiting from PiP
- if apps do want to restore the overlays, it maybe preferred to do that
  in onPictureInPictureModeChanged(isInPictureInPictureMode=false)
  callback rather than the beginning of exit PiP animation

Bug: 303718131
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/dXf5BdzkfR4i2ZNdTLJSaD
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/ho9HTL5X7mneQoJMiHCBYY
Flag: ENABLE_PIP_UI_STATE_CALLBACK_ON_ENTERING
Test: manually, see videos
Test: atest PinnedStackTests
Change-Id: I3137b9c1505443efb633d086fbf6ceee477a4b73