This adds two new scalars for engagements and abandonments in the urlbar:
```
urlbar.engagement
urlbar.abandonment
```
We already have engagement event telemetry but it's preffed off by default, and
for the upcoming best match experiment, data science would prefer scalars so we
can easily measure total engagement volume. (See bug 1752953 comment 22.)
Recording simple scalars for engagements and abandonments in addition to the
optional event telemetry seems totally reasonable.
The existing `urlbar.picked.*` scalars are sort of proxies for engagement, but a
single scalar would make analysis easier, and there is no similar existing
scalar for abandonments.
This revision hooks into the `TelemetryEvent` class, but it records the scalars
regardless of `browser.urlbar.eventTelemetry.enabled` because there's no reason
to not always enable it.
Differential Revision: https://phabricator.services.mozilla.com/D140287
94dbe3ea23