Ensure showWhenLocked works correctly on back-gesture animation

When keyguard is showing and occluded, only do back gesture animation
when previous activity also supports showWhenLocked, otherwise there
should be no animation.

Fixes: 257859919
Test: atest BackNavigationControllerTests
Change-Id: I54bf745a88e43f736fc5f434d923f4017b25c495
2 files changed