metricsd: Fix CumulativeUseTime metrics.

This fixes three things:
* name of the metrics: the metrics does not represent the daily use.
* arithmetic error: we collect the use time in seconds so no need to
  divide by 1000 to get seconds.
* the metrics should not be reset daily.

This CL also renames the Send* helper methods to be more explicit about
resetting the counters.

This was also committed in Chrome OS at:
https://chromium-review.googlesource.com/#/c/300059/.

BUG:24131693
TEST: unit tests.

Change-Id: I712baa1fd639b9b0df60906d94a0904d8c6760aa