ActivityManager: Fix monkey tests

Change I30e38c6c27d339d55a8f9a0f4106185356a9b9a0 (adding ordered
broadcast prioritization) adds the assumption that every broadcast
generator has a full-fledged caller app, which isn't true for
monkey-created tests. Make sure the caller is a full app before
checking if it is AID_SYSTEM to avoid an NPE

Fixes com.android.cts.monkey.*

Change-Id: Ide0dd046c6c5eb10ab4acb504237b58cbbbd8cfa