Enable TLSv1.1/1.2 for API 16-20

Relating to https://github.com/owncloud/News-Android-App/issues/267 (SSL Handshake Error) I enabled TLSv1.1/1.2 for Devices with API 16-20 (at least I think so) since ssl 3.0 is insecure and enable ist is no longer a solution for that problem. 

Please consider that I'm not an Android/Java developer so better assume that I had not a clue what I did there...