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