[android_alarm_manager_plus] View pending alarms

## Use case
Having more than a couple alarms can get confusing to track, so it'd be a good feature to be able to track which and when alarms are going to be executed.

## Proposal
Ability to view upcoming alarms and their parameters like time of their execution, its periodic duration, etc

Start by reading the android_alarm_manager_plus package's existing alarm-scheduling API and the Android AlarmManager behavior it wraps. Determine how upcoming alarms and parameters such as execution time and periodic duration should be represented and exposed; done means callers can inspect the alarms they have scheduled.

Written by the indexing model from the issue text.