selinux: Allow GPU System apps to access gpu config settings

Allowing read and write access will help to read gpu system
config setting from the adreno_config file and help to write
gpu snapshot dump to path /data/misc/gpu.

Below is the avc denial error, being address with this change.
RenderThread: type=1400 audit(0.0:762): avc: denied
{ open } for path="/data/misc/gpu/adreno_config.txt"
dev="sda8" ino=131795 scontext=u:r:system_app:s0
tcontext=u:object_r:system_data_file:s0 tclass=file
permissive=0

Change-Id: I2dbdb2efd925caf534aff2bd2246e21f1a053fe0