android: better handling of round icons

[dwmkerr]

See #71. We now generate round icons - but it might be better to support an icon.round.png image as a template for this, in a similar way to adaptive icon. Then:

$ app-icon init --adaptive-icons --round-icons
$ ls
icon.png
icon.round.png
icon.background.png
icon.foreground.png

The other option would be to simply eliminate round icons and adaptive icons when generating the icons, so that when we are generating icons for projects which have the icons already, we don't get clashes with them.

This also relates to the AndroidManifest, which will explicitly specify if a round icon is actually being used.