Implement notification categories on Android

Problem

Fluxer does not have notification channels (i.e. all notifications are from a single channel). Discord has separate categories for DMs, public messages, incoming calls, etc. so that a savvy user can set their own unique sounds for each category. Currently, there is only one notification channel named "Fluxer".

Proposal

Create a proper implementation of notification channels for the Android app. https://developer.android.com/develop/ui/compose/notifications/channels