Optionally use LogBuffer in ClockRegistry

LogBuffer is nullable here so that it's not a breaking change to
ThemePicker/WallpaperPicker. It's still very useful to see logs
from their instance of ClockRegistry.

Bug: 276759091
Test: Manually checked bugreport
Change-Id: Ic95b273c265c1b32d433ef17da30e9ef9ea1c363