auditallow w_file_perms for mediaserver

The device-independent code only needs read access to sysfs, and this
appears to be enough for at least some devices (Nexus 5).

Bug: 22827371
Change-Id: I3b7b068e98f11f9133f0bdea8ece363e4bd89ae8

mediaserver.te

@@ -34,6 +34,7 @@ allow mediaserver audio_device:chr_file rw_file_perms;
 
 # XXX Label with a specific type?
 allow mediaserver sysfs:file rw_file_perms;
+auditallow mediaserver sysfs:file { write append };
 
 # Read resources from open apk files passed over Binder.
 allow mediaserver apk_data_file:file { read getattr };