Add ability for progress bar to be laid out at top of toolbar

[ekager]

In Fenix some users (mozilla-mobile/fenix#1049) are complaining that with a bottom toolbar and some phones that do not have the Android nav bar, it is difficult to see the progress bar at the very bottom of the display. Adding a flag for the progress bar to be displayed at the top of the toolbar may be helpful.

┆Issue is synchronized with this Jira Task