Unregister various observers in A11yMS#unregisterObservers()

This is just an internal cleanup and a precaution that
AccessibilityManagerService#unregisterObservers unregisters as many
observers as possible so that in tests they won't remain and affect
other tests.

Bug: 421784853
Bug: 418259247
Test: FrameworksServicesTests_accessibility
Flag: EXEMPT test fix
Change-Id: I41e7894711fe5deeecdbfff40e9887a212f58326