SystemUI: fix hidden clock style after a reboot

Selecting hidden clock style and rebooting the device results
in both center and default clocks appearing when you'd expect
to see none.
mClockView is null after a reboot so visibility gets never set.
Set default visibility for clocks to gone.

- AICP'fied.

Change-Id: I4e3e34bef3fdbd1fc29bdb7a6871e51fa33fe7f4