Adds a new hidden preference to enable dismissing file pickers: remote.bidi.dismiss_file_pickers.enabled , defaults to false.
Dismissing file pickers is now only ever possible for sessions handling bidi commands
The FilePickerHandler.allowFilePickers is now a no-op if there was no matching call for dismissFilePickers
Differential Revision: https://phabricator.services.mozilla.com/D276167