Add new PRIORITY_SYSTEM_NAVIGATION_OBSERVER to OnBackInvokedDispatcher

Callbacks registered with the new PRIORITY_SYSTEM_NAVIGATION_OBSERVER priority do not consume back events. Instead they receive the onBackInvoked() back event whenever the system handles a back navigation. This is useful for observing system back navigation behavior without affecting it, e.g., for logging or analytics.

Bug: 362938401
Test: WindowOnBackInvokedDispatcherTest
Test: OnBackInvokedCallbackGestureTest
Flag: com.android.window.flags.predictive_back_priority_system_navigation_observer
Change-Id: I9a483670fdfc31a1b487e8d3d4151c0cc2f7154f