Use callerPackage instead of callerApp.

When AlarmManager sends a broadcast, it only knows the packageName
that requested the broadcast, and the callerApp may not even be
running anymore.  Since we only need the package name for enforcement,
use that instead of the possibly null callerApp.

Bug: 27397019
Change-Id: I8a52d73b4a83eebf85aa1adbfe06bd8886814d4b