Add a periodic update worker for verified domains

This verified domain update worker is only meant to be a reference
implementation is hardcoded to be disabled. Instead of enabling this
worker, it is highly recommended to implement a custom worker that pulls
updates form a caching service and not from websites directly.

Bug: 307557449
Test: presubmit
Flag: EXEMPT external library
Change-Id: If7dbb1617e4add67c581ef0cc301f8ad978b461e