Untrim the address bar value when the user starts manipulating it.
This doesn't untrim on focus, because that would break double-click to select
word, and drag-select. In the future we'll evaluate untrim on focus, but we'll
need additional platform support to replicate that functionality.
The behavior is currently controlled by the feature-gate preference
`browser.urlbar.untrimOnUserInteraction.featureGate`.
Original patch by Marc Seibert.
Differential Revision: https://phabricator.services.mozilla.com/D206135
6e1c1d304a