Change the status bar clock's measuring scheme

This change implements a measuring scheme such that the clock will only
get wider to accomodate wider numbers, but will never shrink until
either:

- The number of characters changes (e.g. going from 12:59 -> 1:00)
- The density or font scale changes

Test: atest ClockTest
Test: manual
Fixes: 163909570
Change-Id: I8382a479f5cf1afb594d1e6238868f76c1220da9