Support for custom sounds on Android

To make use of this, the sound must be placed in the platforms/android/res/raw folder and must be of one of the supported media types (http://developer.android.com/guide/appendix/media-formats.html).

In the push notification payload, specify the sound without the file extension. If your file is named "platforms/android/res/raw/alert.mp3", use "alert" as the name.