VolRef: add initial UT to cover volume APIs

Trying to improve the code coverage by covering all the test related
APIs in AudioService. Currently only trying to obtain a full method
coverage. More tests to come that will cover more branches and more
volume requirements.

Copied test methods from AudioDeviceManagerTest and
DeviceVolumeBehaviorTest into VolumeHelperTest in order to have a
central class that manages all the volume related API tests.

Test: build with EMMA_INSTRUMENT=true EMMA_INSTRUMENT_FRAMEWORK=true SKIP_BOOT_JARS_CHECK=true WITH_DEXPREOPT=false, atest VolumeHelperTest --experimental-coverage
Bug: 323273677
Change-Id: I54b243979b71c03f145808688bb36c702da621e4