Internationalize notification channel name

[PIG208]

When we create the notification channel, the name is hard-coded as "Messages". This is the same name that appears in the user's notification setting:

We could access GlobalLocalizations.zulipLocalizations while ensuring that the app has started (so that it is set to use the right locale), when initially setting up the notification channel. This only happens when using the app for the first time. We might be able to update the name through createNotificationChannel.

Just updating the channel name whenever the app is started is not enough, though. We also want to listen to changes to the locale and update the name whenever that happens. Flutter probably listens to that already so we might find a way to utilize that.

This is a follow-up to:

• Sweep through UI strings to internationalize #277