Do not prepare PB animation if previous activity has no process.

There will be no window nor snapshot if the activity's process is dead,
also the final hierarchy can be unpredictable after the process
restarted.

Flag: EXEMPT bugfix
Bug: 395218543
Test: atest BackNavigationControllerTests
Change-Id: I564299e33ddad134d540b4efaa2d1f00275587b0