unit_tests: Increase date format precision in promo_resource_service_unittest
With the current precision the following tests:
PromoResourceServiceTest.NotificationPromoCompatNoStringsTest,
PromoResourceServiceTest.NotificationPromoCompatPayloadStringsTest,
PromoResourceServiceTest.NotificationPromoTest
are failing if the time zone is set to Beijing.
Reason for failure: China Standard Time (CST), which is GMT+8,
is interpreted as Central Standard Time (GMT-6).
Increasing precision generates unique string for all time zones.
TEST=Run unit_tests using telemetry on Android devices
BUG=358714
Review URL: https://codereview.chromium.org/
221063002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@260977 0039d316-1c4b-4281-b951-d872f2087c98