Label persist audio properties

label all persist.audio.* properties
and allow mediaserver access to them.

Change-Id: If5755d9783dce298e66a25bcb7f17ff17bd83ea7
diff --git a/property_contexts b/property_contexts
index a08ae80..d86bcb6 100644
--- a/property_contexts
+++ b/property_contexts
@@ -29,6 +29,7 @@
 service.adb.root        u:object_r:shell_prop:s0
 service.adb.tcp.port    u:object_r:shell_prop:s0
 
+persist.audio.          u:object_r:audio_prop:s0
 persist.sys.            u:object_r:system_prop:s0
 persist.service.        u:object_r:system_prop:s0
 persist.security.       u:object_r:system_prop:s0