Lockscreen : Hide clock for Chronus/DeskClock

Hide statusbar clock if chronus or the DeskClock widgets
are being displayed

patch set 2 : Add check for Alarm clock widgets
Patch set 3 : Add null checks to be safe
Patch set 4 : More null checks
Patch set 5 : Removed null checks again

Change-Id: I083870b818b6257a3fbdb676c545945438c42fd7