Make TextClock only listen for specific changes.

Bug 68016230

Make TextClock only listen for changes to the 12/24
hour clock setting, so that it doesn't trigger a
change when other settings are updated.

Test: I5dee1694890ec90b0eabb939cd636971ed9ba5d4
Change-Id: Ia345995dc2f019209e3c68e6e8681cd2c9d5f439
1 file changed