Fix dispatchers in tests using settings

SettingsObservers can run their updates async and it's important that
tests provide test dispatcher to Fake*Settings classes so the jobs get
executed when runCurrent is issued.

Bug: 355267965
Bug: 351766769
Flag: TEST_ONLY test fix
Test: enabled registerContentProviderAsync on cuttlefish and ran tests
Change-Id: I6446fb90672cc077a3fff660202ef6388d46c1f0