[expo-notifications] Add NotificationChannelsManager (#6842)

# Why

`NotificationChannelsManager` singleton module will be the manager of notification channels — modules should proxy any changes t

[expo-notifications] Add NotificationChannelsManager (#6842)

# Why

`NotificationChannelsManager` singleton module will be the manager of notification channels — modules should proxy any changes to channels through it. (There are no modules allowing to create, modify or delete notification channels at the moment, so the only thing this module does is providing a fallback notification channel — a channel `expo-notifications` creates for notifications not assigned to any other channel. It's a rather common practice — [FCM does it too](https://stackoverflow.com/a/45930522).)

# How

Added a singleton module, which on appropriate platforms (Android && SDK > .O) tries to create a notification channel reactively (only when asked for a fallback channel). Name is fetched from Android resources, so it will should easy to overwrite or localize by developers.


More complete diagram (receiving + handling + building + side effects)


# Test Plan

I have confirmed that when a new app is installed and no other action is taken, the settings say that the app hasn't created any channels yet. Once `.getFallbackChannel()` is called, settings say that there exists a single _"Miscellaneous"_ channel.

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