Allow sdcardd to read /data/.layout_version

As described in the system/core commit with the same Change-Id,
there's a race condition between installd and sdcard when it
comes to accessing /data/media. Resolve the race by checking
/data/.layout_version to make sure the filesystem has been upgraded.

Maybe indirectly fixes the following SELinux denial:

  sdcard  : type=1400 audit(0.0:3): avc: denied { write } for name="media" dev="mmcblk0p17" ino=102753 scontext=u:r:sdcardd:s0 tcontext=u:object_r:system_data_file:s0 tclass=dir

(cherrypicked from commit 792d8650)

Bug: 16329437
Change-Id: I5e164f08009c1036469f8734ec07cbae9c5e262b