canScheduleExactAlarms returns true for older apps

Callers that don't target S can schedule exact alarms so should get a
return value of true when they call canScheduleExactAlarm.

Test: atest FrameworksMockingServicesTests:AlarmManagerServiceTest

Bug: 191328951
Change-Id: I1cd1d0fb3d3d922360494552e653ed540bfe5227
(cherry picked from commit a7807022)