Fix the recursive call for startBackNavigation.

1. Use a delayed execution so other pending requests in the shell's
main thread have a chance to be executed.
2. Break the recursive request for startBackNavigation if no pending
transition in shell.

Flag: com.android.window.flags.predictive_back_intercept_transition
Bug: 435754613
Test: atest BackAnimationControllerTest BackNavigationControllerTests
Change-Id: I0ef62ce4a6352e906a129014f164a07889f31182