sepolicy: Define new policy rule to read gpu model

Add  a new file context label for gpu_model sysfs entry. allowed read
access to that entry.
Addressing the following denials :
type=1400 audit(0.0:62): avc: denied { read } for
name="gpu_model" dev="sysfs" ino=78734 scontext=u:r:mediaserver:s0
tcontext=u:object_r:vendor_sysfs_kgsl:s0 tclass=file permissive=0
type=1400 audit(0.0:88): avc: denied { read } for name="gpu_model"
dev="sysfs" ino=78734 scontext=u:r:platform_app:s0:c512,c768
tcontext=u:object_r:vendor_sysfs_kgsl:s0 tclass=file permissive=0 app=com.android.systemui
type=1400 audit(0.0:100): avc: denied { read }
for name="gpu_model" dev="sysfs" ino=78734 scontext=u:r:priv_app:s0:c512,c768
tcontext=u:object_r:vendor_sysfs_kgsl:s0 tclass=file permissive=0
app=com.android.launcher3

Change-Id: I9e1b9ffbb88ea62b4cc530564d811d7cfc640bbc