Tidy up changes for timezone update code

Tidy up changes for timezone update code:
1) Remove some TODOs
2) Fix some logging
3) Remove ClockHelper in favor of java.time.Clock /
SystemClock.elapsedRealtimeClock()

No functional changes intended.

Bug: 31008728
Test: PTS: run pts -m PtsTimeZoneTestCases
Change-Id: Ib1ae04cbadfe71e4e340a0572e82f0bc3f68ef30