Define a label for A2DP Media Browser service

In order for applications to render A2DP SNK audio, they may require a
specific label for the streaming service in-stead of relying on the
application label.

Bug: b/28537735
Change-Id: If4e422160802ba578b0f38630b82462fcb619b33
(cherry picked from commit 649af3a18c6acdfbaf84a71f215f309e27c43021)