SystemUI: allow for the clock to be centered (2 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: I4f71263d94128619e3347585e188b29ab3a7f19b