Action icons: Give the Invert icon rounded corners.

With a 2dp radius as specified here:
http://www.google.com/design/spec/style/icons.html#icons-system-icons
though the
"Each system icon size set requires a different round corner for optical consistency."
text there might be trying to say, in designese, that we need a custom SVG
for each icon size.

The PNGs were generated from the SVG with this:
http://romannurik.github.io/AndroidAssetStudio/icons-actionbar.html#source.type=image&source.space.trim=1&source.space.pad=0&name=ic_action_invert&theme=light&color=33b5e5%2C60