Add callback support to FakeSettingsProvider.

Users of FakeSettingsProvider can optionally provide a callback
that will be called whenever a setting's value has changed.
This can be used to implement SettingsObservers.

Bug: 397521796
Flag: TEST_ONLY
Test: new unit tests
Change-Id: Ifebdb41ede706190daa4d7740ceb4f662ae29d25