**Root cause**
Two reasons of the flaky test:
- Settings value not reset to default after tests
- Need time for the settings value changes to propagate to its listeners
Other clean up: When creating a preference, by default it will store
values in the shared preferences. If we don't intent to save the
preference to the sharedPreferences, we should set presistent as false.
Bug: 279818352
Test: atest
SettingsUnitTests:com.android.settings.accessibility.DisableAnimationsPreferenceControllerTest#onStart_disabledAnimation_shouldReturnTrue
--iterations 50
Change-Id: Ie645f8e847fc8ce188a7e5ed45bf5050f81e1d9a
a1fe1ef36e