Mark SysApi methods in AudioDeviceVolumeManager

The methods marked SysApi are necessary to implement the logic for
absolute volume management.

Test: atest AudioDeviceVolumeManagerTest
Bug: 396216663
Bug: 393657380
Flag: android.media.audio.unify_absolute_volume_management
Change-Id: I54bf93e23efe4b9653f68ecb3b5f08c1f2e75366