Use is_first_run exclusively in mobile targeting

[freshstrangemusic]

The mobile targeting context has is_first_run: bool and isFirstRun: string. We should only use former going forward and remove all usages of the latter.

See-also: https://github.com/mozilla-mobile/firefox-ios/issues/22173 See-also: https://bugzilla.mozilla.org/show_bug.cgi?id=1920793

┆Issue is synchronized with this Jira Task