Re-add launcher icons for API 4-

Those were previously removed in 47c753a6 because Gradle does add generated images build/generated/res/pngs//drawable- but apparently they need to reside in the mipmap-* folders instead.

Also removed ic_launcher_round.xml. I don't think it's being used, at least I cannot find any reference to it. Those versions of Android that use the vector drawables set the border to some preferred shape anyway, like circle, squircle or squre with rounded corners or whatever vendor-specifc shape is implemented.

Closes #19 (closed)