refactor webserver class into much simplified NanoHTTPD subclass

The SimpleWebServer.java example had a lot more stuff than we needed in it,
plus it had some code that was only supported on android-9 or higher. I
stripped out anything that is not currently needed and merged
KerplappHTTPD and SimpleWebServer into a single class: LocalHTTPD.

This should now work down to android-7 at least.

refs #3204 https://dev.guardianproject.info/issues/3204
when changing "Use HTTPS", use AsyncTask to keep things responsive

The process of configuring and kicking off the Service to handle the
changes needed can mean that the Settings Activity pause. Putting that into
an AsyncTask ameliorates that.