Split time tracking from network updating functionality in NetworkTimeTracker

The time tracking part will be used in other places for secure time, so
this splits it out into a separate class (TimeTracker), that doesn't
include any of the network updates bits.

This does not yet change any behavior

Bug: 41490083
Change-Id: Iff7498c4e0cace49ae437e925a1c021bc8d7f715
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5798033


Auto-Submit: Carlos IL <carlosil@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Commit-Queue: Matt Mueller <mattm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1343865}