TopLevelStoragePreferenceControllerTest converted to JUnit

Helper function composed to return thread future, to guarantee
concurrent function finishes before test condition is checked.
Additionally a @VisibleForTesting helper function is used rather than
robolectric reflection helpers to set a class variable.

Test: atest -c TopLevelStoragePreferenceControllerTest
Change-Id: Ia22ebac4baec60f72b8516e9e79998cd788266ab