Add Activity#onPictureInPictureUiStateChanged and PictureInPictureUiState.

This adds a new API on Activity such that app developers can get signals
when certain PiP UI states change, e.g. stash/unstashed. The idea is that
when PiP is stashed, the activity will get a signal to do anything they
wish, such as pausing/resuming video-like contents.

Bug: 182409584
Test: atest PinnedStackTests#testPictureInPictureStateChangeCallback
Change-Id: I308f3f1c3af8d1a54b63311fb3beaee77d6c7922