Add support for external volume handler of volume adjustements

For devices that use the config_handleVolumeKeysInWindowManager
  configuration, add API to receive volume key-related events
  directly, without them being processed by the framework
  (already not received by apps).

Bug: b/63906162
Test: set config_handleVolumeKeysInWindowManager, use
    "adb shell dumpsys audio" to verify adjustVolume methods aren't
    logged

Change-Id: I432b14fa9980764d69077b9f1b23b8c95f30814d