cLock: refactorings and cleanup (no functional changes)

Change-Id: I4ca0f3eea0cb3260f248d3abc6099b45c3ed1c27
cLock: check calendar visibility on start of CalendarWidgetService

(see also http://review.cyanogenmod.org/#/c/33973/)

Change-Id: I24e250db779c343b82fe71469827be2bc565a09d
cLock: optimize widget display if parts are not visible

Change-Id: Icc49411b20f27d9ae254ce672e36cb87f2a1602e