SystemUI: Status bar clock - fixes

This commit fixes:

- no overlapping more from status bar icons on center clock.
  The width depending on what the user has setuped (date and clock) is exactly meassured now
- do not start animations if they are not needed
- reduce settings provider calls
- a bit cleanup

Change-Id: I7a37a6ae8f9bad0f265119af05dc20cf21cd6890
SystemUI: Ensure that the Clock is actually enabled in IconMerger

to device if we need to calculate

Change-Id: If12b0e26b32f044f1a82cf8b539283194d1d0b4c