CTS for isInPictureInPictureMode when onPause

When app receives onPause callback, we ensure isInPictureInPictureMode
returns true for both regular and auto PiP cases.

See https://developer.android.com/guide/topics/ui/picture-in-picture for
guidance.

Bug: 239618513
Test: atest PinnedStackTests
Change-Id: Ife281f83ba6e72a4dd2360fb31fc20afadbc905d