Improve Orbot's user experience for handling blocked access to the Tor network

(Re-using text from Nathan here.)

Improve user experience for handling blocked access to Tor network or bridges, and prompt to user to try obfsproxy. The interesting thing that you can do on Android is tell whether the user generally has an "Internet" connection or not, meaning they have good connectivity, an IP address, etc. The state we're looking for is when the Internet is good, but Tor is stuck at 10% bootstrapping, or some other typical fingerprint of Tor network blockage. Orbot has solid bridge/obfsproxy support, but right now it is not exposed to users who don't know about it.