Add test for override non-default close activity callbacks.

Verify register/unregister system override callbacks.

Flag: com.android.window.flags.predictive_back_system_override_callback
Bug: 362938401
Test: atest BackGestureInvokedTest OnBackInvokedCallbackGestureTest
Change-Id: I74ae6086fa3445304ea9d514e48da636748f31b8
2 files changed