Have async FakeSettings methods advance the dispatcher

This avoids having to write a lot of testScope.runCurrent invocations after the underlying code runs async/suspend functions.
It makes testing significantly easier for users.

Bug: 327558308
Flag: TEST_ONLY
Test: atest SystemUITests FakeSettingsTest
Change-Id: I1b4140e11f578f7320a3867346f2718568d07aee