exclude su from app auditallow

su is an appdomain, and as such, any auditallow statements applicable to
appdomain also apply to su. However, su is never enforced, so generating
SELinux denials for such domains is pointless. Exclude su from
ion_device auditallow rules.

Addresses the following auditallow spam:

  avc: granted { ioctl } for comm="screencap" path="/dev/ion" dev="tmpfs"
  ino=10230 ioctlcmd=4906 scontext=u:r:su:s0
  tcontext=u:object_r:ion_device:s0 tclass=chr_file

Test: policy compiles
Change-Id: I2e783624b9e53ad365669bd6f2d4db40da475a16