Introducing a lazy alarm store

Lazily batching alarms should perform better. A flag has been
added to switch the implementation at runtime to be able to experiment
and measure the differences.

Test: atest CtsAppTestCases:android.app.cts.AlarmManagerTest
atest CtsAlarmManagerTestCases
atest FrameworksMockingServicesTests:com.android.server.alarm

Bug: 161497385
Change-Id: I240f4f3c5439d39b3a000ab47c512d2b0bd14f6f