Step #1: Implement a Service.
Step #2: Have the activity send commands to the service in the form of Intents delivered via startService() (except for "stop", which would be a call to stopService()).
Step #3: Have the service handle those commands in onStartCommand() by making appropriate calls on MediaPlayer (be sure to use prepareAsync()).
Here is a trivial little example of this, for play and stop, with stubbed-out actual media usage. Here is a slightly more sophisticated example of this, using a Notification and startForeground() to ensure the player can keep playing.
