Add option to start on player screen

Requested by Jason Gibson.

"If xmp is already running, then i press home and go to another app. Is it
 possible to make it so that when i click the xmp icon on my home screen it
 will go directly to the existing session rather than opening a new one?
 This would be very useful in the car - it's much easier and faster to just
 click the big xmp icon on my car dock home screen than to open notifications,
 scroll, and tap the right one."

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>