convert 12-hour format strings when %P is blank

refs CNVS-12398
fixes CNVS-14778

properly handle 12-hour time format strings when the localization
implies a 24-hour string should have been used. using a 12-hour format
when the %P formatter is blank leaves ambiguity as to whether, for
example, "3:00" means 03:00 (am) or 15:00 (pm).

test-plan:
 - set locale to portuguese
 - when an afternoon hour is localized in portuguese, it should display
   in 24-hour format (e.g. "<date> em 15:00" vs. "<date> em 3:00").

Change-Id: I0fa69fb179f1f7eab0da8205ec70d48752021351
Reviewed-on: https://gerrit.instructure.com/33494
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Steven Shepherd <sshepherd@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>