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