AudioFlinger: Extract inner VolumeInterface class

Change to RefBase for proper lifetime as the sibling
interfaces are also refcounted.

Test: atest audiorecord_tests audiotrack_tests audiorouting_tests trackplayerbase_tests audiosystem_tests
Test: atest AudioTrackTest AudioRecordTest
Test: YouTube Camera
Bug: 288339104
Bug: 290288893
Merged-In: I92ad5ce31152813e935cb3e549bde3d11241ef64
Change-Id: I92ad5ce31152813e935cb3e549bde3d11241ef64
4 files changed