Fix flaky VibrationSettingsTest

Update VibrationSettings to expose all listener and observers for
testing.

Change test class to use mocks for internal services, including activity
manager singleton internal service, to avoid flakiness during test runs.

Make all listener/observer tests deterministic by directly triggering
them from the test class.

Change-Id: I95481acb340a27ef115f2605cc2cb7ed2e668b8a
Fix: 381725186
Test: VibrationSettingsTest
Flag: EXEMPT refactor
Merged-In: I7618b7894fa661d1e657c306557717979cbdedbf