Define policy for "loop-control" device.

Per loop(4), this device is the preferred way of allocating new
loop devices since Linux 3.1.

avc: denied { read write } for name="loop-control" dev="tmpfs" ino=15221 scontext=u:r:vold:s0 tcontext=u:object_r:device:s0 tclass=chr_file permissive=0

Bug: 34903607
Change-Id: I1f5f62cf0a1c24c6f6453100004812af4b8e1503