QrWizardWifiNetworkActivity enables wifi and starts the webserver

This makes it less button clicks and things to remember when going through
this process.
move all QR Code image generation into AsyncTask to keep things snappy

Generating the QR Code image can take some time, so do it in an AsyncTask
which then updates the ImageView once the QRCode is complete.

refs #3201 https://dev.guardianproject.info/issues/3201