Avoid leaking Activity from OnBackInvokedCallback.

- Remove mTopCallback field
- Cleanup compat and default callbacks
- Remove the callback held by WindowState when the window is removed.

Bug: 218328707
Test: atest packages/apps/Launcher3/tests/src/com/android/launcher3/ui/TaplTestsLauncher3.java
Test: atest frameworks/base/core/tests/coretests/src/android/window/WindowOnBackInvokedDispatcherTest.java
Change-Id: I38fd760a617500c874aef10bc8de165880837008