notif: Use Zulip's notification sound on Android

[rajveermalviya]

Updates #340

For now, only includes the default sound, i.e. it doesn't show multiple sound options in notification settings - this will be done in later PRs.

[gnprice]

Rather than add more parts of the package:flutter_local_notifications API that we use, let's use Pigeon for this. That way in order to be confident of understanding what the code is doing, we only have to read our own code and the Android docs, and not also the flutter_local_notifications code 🙂 (which often introduces a lot of logic of its own).

As a concrete strategy, let's put this issue after #351: so we convert the existing call sites first (the remaining ones, after #592 covered the most complex of them), and then we go on to add Pigeon bindings for more of the notifications API.