Add smart space to custom clock faces.

This changes adds a hook to the ClockPlugin to allow the
custom clock to control the position of the smart space.

Still todo:
 - move out of the way for the lock icon
 - don't slide smart space down on lock screen when no
   notifs are showing

Bug: 129348218
Test: Manually checked custom clock faces.
Change-Id: I2472f6b2bb54f94e2e673f01c78170c0e7f87627