grant appdomain rw perms to tun_device

Previously granted to only untrusted_app, allow all apps except
isolated_app read write permissions to tun_device.

avc: denied { read write } for path="/dev/tun" dev="tmpfs" ino=8906 scontext=u:r:priv_app:s0:c512,c768 tcontext=u:object_r:tun_device:s0 tclass=chr_file

Bug: 26462997
Change-Id: Id6f5b09cda26dc6c8651eb76f6791fb95640e4c7