backnav2: fixes to cross-activity flow in BackNavigationController

- Minor refactor to move screenshotting logic to prepare for a cache
  implementation.
- Prepare animation for CROSS_ACTIVITY type.

Test: atest
WmTests:com.android.server.wm.BackNavigationControllerTests#backTypeCrossActivityWhenBackToPreviousActivity
also tested locally with other changes

Bug: 207481538
Change-Id: I5b5cd35fd0a80f4ac0e03e07f86e15619db092e6