Start locking down /data/media access.

The new sdcardfs filesystem requires that we have stricter access
controls around /data/media style locations.  Start by taking away
the "media_rw" GID from apps requesting the WRITE_MEDIA_STORAGE
permission.

Common use-cases like music playback appear to continue working fine,
as clients should only be attempting to use /data/media paths after
calling maybeTranslateEmulatedPathToInternal().

Test: builds, boots, music playback works
Bug: 35447080
Change-Id: Iba9f3ef41d3277c75497f675a1fe6d3406cf4542

data/etc/platform.xml

@@ -60,7 +60,6 @@
     </permission>
 
     <permission name="android.permission.WRITE_MEDIA_STORAGE" >
-        <group gid="media_rw" />
         <group gid="sdcard_rw" />
     </permission>