Ensure no more callback invocations after onBackCancelled

When unregistering a back callback which is currently in-progress, onBackCancelled is dispatched. After dispatching onBackCancelled, it must be ensured that no more callback invocations to that same callback happen.

Bug: 351777537
Flag: com.android.window.flags.predictive_back_system_anims
Test: atest WindowOnBackInvokedDispatcherTest
Test: Manual, i.e. unregistering a callback during an active back gesture in a test app and verifying that after onBackCancelled no more onBackProgressed or other events are dispatched
Change-Id: I9a0b3a7c19e565070abc550cd02b520c693c443e