Investigate Android compatibility

[gaul]

In #1 @snorp reports:

I needed some other changes to make it work with Jetty 7.x (because 8.x and 9.x don't run on older Android), so probably not.

[gaul]

It might be worth investigating the AsyncAndroid-based approach that Mozilla now uses:

https://phabricator.services.mozilla.com/D56993

java-httpbin should not require much from the environment so it's possible that Jetty is overkill.