Bug 1561245 - Add a null check for the received Intent; r=VladBaicu

According to the documentation -
https://developer.android.com/reference/android/app/Service.html
"The Intent supplied to Context.startService(Intent) may be null if the service
is being restarted after its process has gone away, and it had previously
returned anything except START_STICKY_COMPATIBILITY."

We will ignore the case when the system restarts our Service as we only expect
it to be properly started by us, in the event of a crash.

Differential Revision: https://phabricator.services.mozilla.com/D35790

--HG--
extra : moz-landing-system : lando

mobile/android/base/java/org/mozilla/gecko/CrashHandlerService.java

@@ -8,6 +8,7 @@ import android.content.Context;
 import android.content.Intent;
 import android.os.IBinder;
 import android.provider.Settings;
+import android.support.annotation.Nullable;
 import android.util.Log;
 
 import org.mozilla.gecko.notifications.NotificationHelper;
@@ -22,8 +23,12 @@ public class CrashHandlerService extends Service {
     private static final int ANDROID_Q = 29;
 
     @Override
-    public int onStartCommand(Intent intent, int flags, int startId) {
-        if (ACTION_STOP.equals(intent.getAction())) {
+    public int onStartCommand(@Nullable Intent intent, int flags, int startId) {
+        if (intent == null) {
+            // no-op
+            // The Intent may be null if the service is being restarted after its process has gone away,
+            // and it had previously returned anything except START_STICKY_COMPATIBILITY.
+        } else if (ACTION_STOP.equals(intent.getAction())) {
             dismissNotification();
         } else {
             // Notify GeckoApp that we've crashed, so it can react appropriately during the next start.