Show Tor Browser version on Android

It's is currently not easy to check the Tor Browser version on mobile. The about:tor page on desktop has the version number on the upper right corner but there is no equivalent for mobile yet.

The second piece that is confusing here is mentioned in https://blog.torproject.org/comment/282625#comment-282625 where the Firefox version is shown in about -> Tor Browser on the Settings menu. I guess we should keep the Firefox version as we do on the about Tor Browser dialog on desktop but amend it with the Tor Browser version (which we do as well). Might be a good child ticket.