Let Settings measure transient free space.

Transient volumes like USB drives are only mounted at /mnt/media_rw,
but they still appear in Settings > Storage.  To show stats like
free/used space, give Settings the permissions it needs to access
devices mounted there.

avc: denied { search } for name="media_rw" dev="tmpfs" ino=8358 scontext=u:r:system_app:s0 tcontext=u:object_r:mnt_media_rw_file:s0 tclass=dir permissive=0

Bug: 22545248
Change-Id: I273a1729e417873184ad04ba9dd0fec95fd54f97

system_app.te

@@ -27,6 +27,9 @@ allow system_app misc_user_data_file:file create_file_perms;
 auditallow system_app system_data_file:dir { create setattr add_name remove_name rmdir rename };
 auditallow system_app system_data_file:file { create setattr append write link unlink rename };
 
+# Access to vold-mounted storage for measuring free space
+allow system_app mnt_media_rw_file:dir search;
+
 # Read wallpaper file.
 allow system_app wallpaper_file:file r_file_perms;