A WorkSource version of AlarmManager#setExact

Inexact alarms will have a minimum length window enforced starting with
API S, so we need a clearer separate system API for apps that set alarms
on behalf of others.

Test: atest CtsAlarmManagerTestCases:BasicApiTests

Bug: 185530825
Change-Id: Ica8399b21421572e30a554f9897a352ce5fff557