Stop screen off timer when receiving ACTION_USER_PRESENT or SCREEN_ON_ACTION

The screen off timer was not stopped, if "enable wifi on unlock" was not
set. Therefore the wifi was turned off, even if the screen was disabled
only a few seconds.
Changed preferences were not used immediately

The 'Receiver' service did not always use the updated preferences
immediately
Only start timers when not already running

If the screen was turned off and then on again without unlocking the
screen, the timer was not stopped (since on ACTION_USER_PRESENT was
sent), but restarted when the screen was off again.
Fixed comment
Merge pull request #9 from friedlP/master

Fixed some problems with the screen off timer