Add clock custom fragment

Add clock custom fragment for customizing clock surface.
This is just an empty UI and in need to call clock face's API for the
content.
Design: http://shortn/_7RDNjpWEmS

Bug: 241966062
Test: https://screenshot.googleplex.com/AUX4So9hYia6SjD
Change-Id: Id3cc910a2b181e88a01dd2f4a4d00eb54de0b10c