SystemUI: allow for the clock to be centered (1 of 2)

This allows the clock to be centered and does
not break the normal functionality of the status bar.
Overflow for status bar icons is adjusted accordingly
when the clock is centered. The clock slides in/out
when centered and the ticker is running.

Change-Id: I8bb99af278d316d7632cd07ff5c4722c5a27cb12
Merge remote branch 'cm/cm-11.0' into cm-11.0