Small refactor AudioDeviceVolumeManager in AudioService

Address comments in http://ag/29644899: renaming, changing public to
private, small enhancements, etc.

Change-Id: I7cab332b1b50dac546a25140b86ae3ec6f8e0b02
Bug:b/364923030
Test:AudioServiceTest
Flag:com.android.media.flags.enable_audio_input_device_routing_and_volume_control