lockscreen reorganization

our lockscreen options were starting to look a bit cluttered so to clean it up:

- widget options moved to their own class and preference screen
- lock clock preference moved to lockscreen options
- all widget options including lock clock are now dependant on widgets being enabled/disabled

Change-Id: I0fe0169d87c40704148d93cb0be43c17064c036b
remove unused imports

Change-Id: I2f34c8d3db4e0c38965fa235582243578a4e23ae