[2/n] Independent back per display in projected mode

In projected mode, system back should affect only apps in the display
that the back originated from, even if the app is not currently the top
focused window.

Flag: com.android.window.flags.enable_independent_back_in_projected
Bug: 401530192
Test: atest BackNavigationControllerTests
Change-Id: I6ef243c0bda9097a61a1da6be9b86895793075e3