Add an API for a prioritized alarm

Adding an API that system apps can use to get alarms while the device is
in doze. These alarms will be prioritized over other alarms in the sense
that the device may wake up to send these alarms and ignore other
pending due alarms at that time.

Test: atest CtsAlarmManagerTestCases
atest FrameworksMockingServicesTests:com.android.server.alarm

CTS-Coverage-Bug: 183661625

Bug: 182843915
Change-Id: Iccae4f532bf145f4e0b130241ca15ff0ee0f106a