Change payload message as alert.

When building cross platform unify push server backend, usually we would like to standardize backend API into unified format, similar as AeroGear's RESTful API:
- http://aerogear.org/docs/specs/aerogear-unifiedpush-rest/sender/index.html

Refer to AeroGear's suggestion about unify developer experience (http://aerogear.org/docs/unifiedpush/push-message-format/#cordova-android-special-keys):
> Cordova Android special keys
> To make the user experience the same on iOS and Android, for cordova users, we use the iOS alert ‘key’ on Android as well to generate a notification for you. And we’ve introduced a ‘title’ that can optionally be used for the title of this notification, if none is specified the application name will be used like on iOS.

This patch simply modify the payload's key from hardcoded "message" to "alert", so sharing a similar experience as like as IOS for Android.