Add required null-checks while handling network intents.

Performing the required null-ckecks in the GpsLocationProvider while
handling intents, related to Network-Initiated requests, that are not
protected on AOSP, prevents a malicious app to trigger NPEs in the
System Service, and consequently making possible a DoS attack.

Bug: 20917373
Change-Id: I417352a800a7706377174500c78cb0b4dd71ff81