Refactor WeeklyTime to be explicit with timezones

* Add timezone_offset_ member variable to WeeklyTime class, this
will help the class users know which timezone the class is to
be interpreted in.
* Add ConvertToTimezone and ConvertToCustomTimezone functions to change
between well defined timezones and also to set the timezone of a time
agnostic WeklyTime.
* Move ToLocalizedString to time_utils since its a more specialized
use case.
* Add unit tests for timezone related functions to time utils.

BUG=chromium:852860
TEST=unit_tests

Change-Id: Ifef1b17471c30fb7443727323d00523193327ef1
Reviewed-on: https://chromium-review.googlesource.com/1155786
Commit-Queue: Adolfo Higueros <adokar@google.com>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581420}
16 files changed