Make private browsing mode clear to TalkBack users

[sblatz]

Follow-up to #804:

From @eeejay

We need to figure out a prominent way for TalkBack users to know they are in private browsing. Right now, they would need to check the state of this button or read the private browsing copy in the private browsing home screen.

┆Issue is synchronized with this Jira Task

[mheubusch]

@eeejay do we need to have actual text present on the screen in the address bar area or can we handle by updating the alt text of the PB button to say Private Browsing Mode On or is there other alt text we deploy to show state change from normal to private mode?

[cpeterso]

Moved to bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1807556

Change performed by the Move to Bugzilla add-on.