test/fake_osi: Improve fake alarms managing

With this patch, alarm_is_scheduled() will return based on scheduler
alarms and not only based on last scheduled alartm.

Also some loging has been added.

Bug: 372120241
Flag: TEST_ONLY
Test: mmm packages/modules/Bluetooth
Change-Id: Id4b77fd0eaf2a771546a99183692fed55988a919