Do not merge. Register a callback to push volume updates in MediaSessionCompat

We had forgotten to set a listener for volume changes on in
MediaSessionCompat when a VolumeProviderCompat is set. Without this
future volume changes won't be reported correctly to observers on pre-L
devices.

bug:18959923
Change-Id: I3b136c6bf923b3a2e30e7894fffb38feccd206c6
1 file changed