Handle volume keys to apps in Vr mode

When a device is in Vr mode -
 - Volume key event must be passed to the audio subsystem to reflect
 changes when the configuration is enabled.
 - When the configuration is disabled, neither the apps not the audio
 subsystem should be able to receive the volume key events

Bug: 37625887
Bug: 36280728
Test:
- Verified that the app doesn't receive volume events
- Verified that the system volume bar is also not triggered

Change-Id: I245f7797873f764d0b017879723a6dbff1bb4d36
Signed-off-by: Karthik Ravi Shankar <karthikrs@google.com>