Ensure isInPictureInPicture result in onPause

We use to set Activity#mIsInPictureInPictureMode to be true when app
requests Activity#enterPictureInPictureMode and therefore when they
receive the onPause callback, Activity#isInPictureInPictureMode can be
true. With the new auto-enter-pip API, this code path is no more
relevant and here we add extra info to PauseActivityItem to set the same
flag when WM puts the Activity into PiP mode due to auto-enter.

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/yXi24xi6TgYGhoqB31Oov
Bug: 239618513
Test: follow the reproduce steps in bug, see video
Test: atest PinnedStackTests
Change-Id: I81ece7d3aad8eb868d95f106f6825612e0741a19