Add AvrcpVolumeManagerTest

Also adds some test methods in BatteryServiceTest.

Bug: 237467631
Test: atest AvrcpVolumeManagerTest;
      atest BatteryServiceTest;
Change-Id: I01992e5f289bce2dfeaabfe644efdcc507a8555a
(cherry picked from commit d1130c81)
Merged-In: I01992e5f289bce2dfeaabfe644efdcc507a8555a