Disable custom clock faces in SystemUI

This will prevent custom clocks from persisting on dogfooders devices.

Bug: 136090066
Test: `adb shell settings put secure lock_screen_custom_clock_face
"com.android.keyguard.clock.BubbleClockController"` doesn't switch to
Bubble clock.

Change-Id: If9fc43295380d0bc37ca3df4c7187bd4faaf4bde