Disallow enterPip from auto-pip-able while pausing

When an app supports auto-enter-pip and requests
Activity#enterPictureInPictureMode while pausing, return false
immediately from the system server.

Such practice, from the app, is not expected and it's against the idea
of auto-enter, which can also cause race conditions.

Bug: 212636597
Test: atest PinnedStackTests
Test: atest ActivityTaskManagerServiceTests
Change-Id: I4e0fca7e83743f5c3edd4bd292f07a11f6ffd441