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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:4fad1456409b79d6e649a29d5116a4fe3160bd21)
Merged-In: I458a9508662e72a1adb9d9818105f2e9d7096d44
Change-Id: I458a9508662e72a1adb9d9818105f2e9d7096d44