allow media.metrics to write to file descriptor in /data

when GMSCore collects information for uploading via 'dumpsys
media.metrics', it provides the metrics service with a file descriptor
to hold the data, which is post-processed by the log collecting code
in GMScore.  This temp file lives in /data/, a place where our
restrictive policies for media.metrics doesn't allow any writing.

This relaxes the restrictions -- allowing media.metrics to
write to already open file descriptors it might be given on that
partition.

Bug: 36660639
Test: build/boot
Change-Id: Icbaa76b04ecf731014b6b1bb1283bc2951a6ae4b

public/mediametrics.te

@@ -14,6 +14,9 @@ allow mediametrics system_server:fd use;
 r_dir_file(mediametrics, cgroup)
 allow mediametrics proc_meminfo:file r_file_perms;
 
+# allows interactions with dumpsys to GMScore
+allow mediametrics app_data_file:file write;
+
 ###
 ### neverallow rules
 ###