Injecting system calls into alarm manager

Injecting and mocking dependencies to make the service unit testable.
Shuffled around initialization to make that possible.

Also added the first batch of unit tests. The idea is to have extensive
coverage of minute pieces of alarm manager logic, which should help us
catch any regressions caused by future changes to the code.

Test: atest com.android.server.AlarmManagerServiceTest
atest CtsAlarmManagerTestCases

Bug: 111454659
Change-Id: Ifa1c09646f688cf5c41abe17d98dbc19978eac70