Add Android helper function to get 'google_app_id' parameter.

- This is pretty ugly code. I welcome refactoring from someone more familiar with Android best practices.
- Closest StackOverflow I found was http://stackoverflow.com/questions/24453018/plugin-preferences-in-config-xml

src/android/com/adobe/phonegap/push/PushConstants.java

@@ -54,4 +54,5 @@ public interface PushConstants {
     public static final String GCM = "GCM";
     public static final String CONTENT_AVAILABLE = "content-available";
     public static final String TOPICS = "topics";
+    public static final String GOOGLE_APP_ID = "google_app_id";
 }

src/android/com/adobe/phonegap/push/PushPlugin.java

@@ -40,6 +40,29 @@ private Context getApplicationContext() {
         return this.cordova.getActivity().getApplicationContext();
     }
 
+    /**
+     * Retrieve the senderId we added to AndroidManifest.xml when we installed.
+     *
+     * @return senderId String
+     * @throws NameNotFoundException
+     */
+    private String getSenderIdFromApplicationManifest() throws NameNotFoundException {
+        // Ugly boilerplate to get the metaData from the AndroidManifest. Refactoring welcome!
+        Context context = getApplicationContext();
+        ApplicationInfo app = context.getPackageManager().getApplicationInfo(context.getPackageName(), PackageManager.GET_META_DATA);
+        Bundle bundle = app.metaData;
+
+        // We cannot getString() because ID is a number. We cannot getInt() because the senderId
+        // has 13 digits & the max length of Android int is 10 digits. We must therefore get a float.
+        // See similar StackOverflow if you don't believe me
+        //   http://stackoverflow.com/questions/22210404/android-metadata-from-manifest-returning-null
+        Float senderIDAsFloatObj = bundle.getFloat(GOOGLE_APP_ID);
+
+        // There is no getLong(). We must do the convoluted & somewhat unsafe conversion.
+        //   http://www.tutorialspoint.com/java/lang/float_longvalue.htm
+        return Long.toString(senderIDAsFloatObj.longValue());
+    }
+
     @Override
     public boolean execute(final String action, final JSONArray data, final CallbackContext callbackContext) {
         Log.v(LOG_TAG, "execute: action=" + action);