[clock] use `tnum` instead of custom onMeasure for Clock.java

Tabular font figures was the real friends we made along the way.

On a more serious note. The custom `onMeasure` was built to avoid the
problem that the clock would subtly change width on every minute change.
This problem is even-worse if seconds are showing.

This change requests that the Clocj use a tabular numeral style, which
is a more font-built-in method for solving the same problem.

Test: manual
Bug: 372549909
Flag: EXEMPT bugfix
Change-Id: I0a8b5fe192356e04a9704ef3e76648a4786e1b91