Allow stat of /system/bin/app_process by zygote.

This resolves denials such as:
type=1400 audit(7803852.559:251): avc:  denied  { getattr } for  pid=5702 comm="main" path="/system/bin/app_process" dev="mmcblk0p25" ino=60 scontext=u:r:zygote:s0 tcontext=u:object_r:zygote_exec:s0 tclass=file

(triggered on an art crash seen in recent AOSP master)

Rather than just adding this permission individually, just rewrite
the existing rule to use the rx_file_perms macro.
We already allowed most of these permissions by way of the
domain_auto_trans() rule via init_daemon_domain() and the rule
for the --invoke-with support.  Using macros helps reduce
policy fragility/brittleness.

Change-Id: Ib7edc17469c47bde9edd89f0e6cf5cd7f90fdb76
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>