change runApp command to use mozApps API and app origin

Change simulatorActor runApp command to use app origin to find and launch an installed app.
This change prevents the b2g-instance to exit if the application is not currently installed and doesn't launch the wrong app if you app share the same name.

 NOTE: runApp doesn't always show the app when the simulator instance
 is not already running, because the homescreen is not yet fully loaded and
 the app will start in background (reachable by pressing the home button to reach
 the gaia tasks list)