Add function to check for AVRCP target to send volume keys.

Currently, when A2DP is connected and the user changes volume
keys on the device, we just change the volume on the device.
We are not supposed to do that. The white paper recommendation
is to keep the volume stream at max and just send the volume
keys over. But then not all A2DP headsets act as AVRCP targets,
hence check for that.

So now the media code should use the API and not change the volume
at the device but just send the keys.

Bug: 3071356
Change-Id: I03a8b6f7619cf860c4e63979bf4903ded9ecd314