This uses the existing `browser.launched_to_handle ::
system_notification` event. This is expedient and looks ahead to
making the WDBA a background task, where we likely will set the
privileged `name` of the toast to `default-browser-agent` or similar.
The alternative is to add a `browser.launched_to_handle ::
default_browser_agent` event. I started with this and it's simply
duplication. The data analysis phase will look almost identical with
either implementation: it's either filtering on the event name or on
the `name` key in the event extras.
Differential Revision: https://phabricator.services.mozilla.com/D179257
76a8b8897c