Fix FrameworksServicesTests crash in A11yManagerService

Currently, A11yContentObserver keeps observing settings value changes
even after the test ends.
This fixes by unregistering it on tear down.

Bug: 418259247
Test: FrameworksServicesTests_accessibility
Flag: EXEMPT test fix
Change-Id: I8a07495479c4c798e93d9b881c8f2648373ac542