Refactor methods of FadeOutManager

The target is to eventually make the FadeOutManager
properties configurable. As a first step:
1. Update the interface to include common method calls
2. Use the interface instead of static methods
3. Convert synchronized methods to using lock object
4. Add new unit test for the class

Bug: 186905459
Bug: 302870089
Test: atest -c --rerun-until-failure 1000 AudioFocusTest
            FadeOutManagerTest

Change-Id: I00f7986ccfeeac690035d390e9aa1a1c87db61e7