Continue work for https://chromium-review.googlesource.com/c/chromiumos/platform/factory/+/686335/

Stimim and I reviewed the time_sanitizer plugin and found that it's not enabled by default today (periodic=0) , and even if we enable it, it's still probably not very helpful.

Currently time sanitizer will try to reach factory server, and retry after periodic_secs, and will stop syncing if it has synced successfully one time.

However, for most factories, DUT will boot without network, and will only be connected for certain tests (for example sync_factory_server). This makes the sanitizer plugin useless because it's hard to set periodic_secs.

As a result, we think the plugin should be changed to wait until network is considered as available (should be notified instead of polling), start the sync, then stop.

Also, using htpdate is better than the poor python implementation.