update proguard config to not strip needed classes

This stops the stripping of classes required for the swap stuff. It also removes -dontobfuscate since the bugfix for #372 (closed) is supposed to be based on changing the name of the classes, i.e. obfuscating them. https://gitlab.com/fdroid/fdroidclient/issues/372

I used apktool to decompile APKs then compare the unpacked folders using meld. That shows that the obsfuscation was not happening, since the classes all had their original names.

fixes #391 (closed) https://gitlab.com/fdroid/fdroidclient/issues/391