Fixes #14678

This is a WIP and it's not ready to merge yet, as it depends on #14698.

This PR adds all the logic necessary to schedule and show reminder notifications using WorkManager. The main classes added here are:

• ReminderConfig, which has all the logic to create daily and weekly reminders.
• ReminderScheduler, which uses a ReminderConfig to create a WorkManager request.
• ReminderNotifier, which is responsible for building and showing a blogging reminder notification.
• ReminderWorker, which is the actual worker implementation that will call both the ReminderNotifier and ReminderScheduler (to schedule the next reminder).

A few things to note:

• Even though the WorkManager framework supports creating periodic works, these are not really suitable for our use case here. The reason is that we can never control the exact time in which a work would be triggered, we can only control its initial delay and the interval between each work. And since WorkManager respects things like Doze and battery management, we don’t know for sure at which time the following works would be triggered. So if our work is delayed a little bit every time, this means that over time the notification would appear much later than before. This video explains this in greater detail and suggests following the approach we're using here, which is to schedule a one time work that schedules another one time work by the end of it.
• There are many things that were done in Setup for local push notifications #14691 that could probably be put to good use here. I tried to refactor this multiple times in order to achieve a more generic solution for local notifications, but there were several different requirements here, mostly related to things that we need to do inside the worker (checking if we can show a notification, build a notification and schedule the next work) that eventually required larger changes and, in the end, I wasn't happy with the direction it was taking. I'm thinking that maybe we shouldn't share the same worker for different types of local notifications as each of them would probably have different requirements, and the WorkManager framework already provides us tools to handle this. But this is something we can discuss later. For now, I've left this PR as simple as possible, just so we can move on with it.

To test:

Follow the instructions in #14720.

Regression Notes

1. Potential unintended areas of impact

N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)

N/A

3. What automated tests I added (or what prevented me from doing so)

Unit tests for newly added classes.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.