mobile: bind the power off key to "Turn Off Screen"

PowerOff key is currently bound to "PowerOff" action which then have 3
possible actions,

- Turn Off screen
- Show logout dialog
- Suspend session

This is super complicated and complicates the code elsewhere when on
mobile you want to allow turning screen off but not shutting down
action.

So simply on mobile, bind the Power key to Turn Screen Off action
instead of the PowerOff action