Fix use of valgrind via app wrapping

On userdebug / eng builds, Android supports the concept of app wrapping.
You can run an app wrapped by another process. This is traditionally used
to run valgrind on apps, looking for memory leaks and other problems.

App wrapping is enabled by running the following command:

  adb shell setprop wrap.com.android.foo "TMPDIR=/data/data/com.android.foo logwrapper valgrind"

Valgrind attempts to mmap exec /system/bin/app_process, which is being denied
by SELinux. Allow app_process exec.

Addresses the following denial:

  <4>[   82.643790] type=1400 audit(16301075.079:26): avc:  denied  { execute } for  pid=1519 comm="memcheck-arm-li" path="/system/bin/app_process32" dev="mmcblk0p25" ino=61 scontext=u:r:untrusted_app:s0 tcontext=u:object_r:zygote_exec:s0 tclass=file

Bug: 15146424

Change-Id: I65394938c53da9252ea57856d9f2de465bb30c25