Allow adbd access to gpu_device.

Addresses denials seen when attempting to take a screencaputre from ddms:
<5>[ 1232.327360] type=1400 audit(1393354131.695:41): avc:  denied  { read write } for  pid=18487 comm="screencap" name="nvhost-ctrl" dev="tmpfs" ino=4035 scontext=u:r:adbd:s0 tcontext=u:object_r:gpu_device:s0 tclass=chr_file

Bug: 13188914
Change-Id: I758e4f87ab024035604d47eebae7f89f21ea1e3e