Add compatibility back callbacks.

For pre-T apps, the default back callback dispatches KEYCODE_BACK to the
application's root view.
For T+ apps, the default KEYCODE_BACK listener invokes the registered
OnBackInvokedCallback.

Test: m -j
Bug: 195946584
Change-Id: I62342b249e6cd6f8b036f7e950cc3f9754418ee4