Remove Activity if it enters PiP without window

This is to prevent malicious app entering PiP without being visible
first, like blocking onResume from completion. Which in turn
leaves the PiP window in limbo and non-interactable.

Bug: 265293293
Test: atest PinnedStackTests
Change-Id: I458a9508662e72a1adb9d9818105f2e9d7096d44
1 file changed