Create OnBackInvokedCallbackInfo to wrap callback and its priority.

Main motivation is to store a back callback's exact priority value in WM. This is required by the IME migration (ag/17076160) to compare the priority levels of IME window callback and focused window callback in BackNavigationController.

This also consolidates the WindowState#mSystemOnBackInvokedCallback and WindowState#mApplicationOnBackInvokedCallback fields into one field, as tracking two fields for one callback was error prone. We had to remember to clear the application / system field when the other field is set, and failing to do so has resulted in bugs such as b/222675481.


Bug: 224856664
Test: atest BackNavigationControllerTest
Test: atest WindowOnBackInvokedDispatcherTest
Test: m -j and test back behavior throughout the system on apps that
opted in and out.

Change-Id: Ic57113610d934f33d2c9ca4cef59f39a9b87e832