Refactor SENDER_ID preference's use.

- Move the preference to just Android Platform.It is not used for iOS. I suspect this resolves phonegap#693
- Add SENDER_ID to AndroidManifest. This exposes the value to the Android code. While 6 years old, http://blog.iangclifton.com/2010/10/08/using-meta-data-in-an-androidmanifest/ describes the exact use case we are trying accomplish.

plugin.xml

@@ -26,11 +26,11 @@
         <engine name="cordova-ios" version=">=4.0.0" />
     </engines>
 
-    <preference name="SENDER_ID" />
-
 	<!-- android -->
 	<platform name="android">
 
+        <preference name="SENDER_ID" />
+
 		<config-file target="res/xml/config.xml" parent="/*">
 			<feature name="PushNotification" >
 				<param name="android-package" value="com.adobe.phonegap.push.PushPlugin"/>
@@ -52,9 +52,9 @@
 		</config-file>
 
 		<config-file target="AndroidManifest.xml" parent="/manifest/application">
-			<activity android:name="com.adobe.phonegap.push.PushHandlerActivity" android:exported="true"/>
+			<meta-data android:name="google_app_id" android:value="$SENDER_ID" />
+            <activity android:name="com.adobe.phonegap.push.PushHandlerActivity" android:exported="true"/>
             <receiver android:name="com.adobe.phonegap.push.BackgroundActionButtonHandler"/>
-
             <receiver
                 android:name="com.google.android.gms.gcm.GcmReceiver"
                 android:exported="true"