Use single snapshot when predictive back opens two adjacent activities

For predictivie back, when previous target are two adjacent activities,
use the snapshot or splash screen to occludes them like normal starting
window. When gesture trigger and close transition ready, reparent the
starting surface to task, so the starting surface can play remove
animation.

Bug: 274997067
Test: launch two adjacent task fragments, launch another fullscreen
activity, trigger predictivie back. Verify no flickering when either
trigger gesture back or cancel.
Test: atest BackNavigationControllerTests

Change-Id: I03cafe39c7ad99e4c76be6c325a45f4c2b52cb26