Batch download notifications into one notification or implement priority queue

[sblatz]

In #5285 we implemented a partial solution for handling Android's silent failure when too many notifications are sent in quick succession.

Chrome does something pretty smart here by implementing a priority queue so that notifications with PAUSE or RESUME are considered "high priority" and always processed.

As discussed with @pocmo we could also just batch the downloads into a single notification to make managing them easier.

┆Issue is synchronized with this Jira Task

[csadilek]

Moved to bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1792945

Change performed by the Move to Bugzilla add-on.