Cleanup mw ui checkbox margins

Cleanup mw ui checkbox margins

The margin right should apply to the checkbox itself to create a
gap between the pseudo element checkbox and the label.

The existing setup causes rendering issues on io8 and is unnecessary
here. The purpose is to create a space for the checkbox. margin-right
doesn't make any difference to an absolutely positioned element and applying
margins to the label interferes with the pseudo elements themselves.

Bug: 72727
Change-Id: I72af4a53e22b1adb3a70af85f5453a1bc9830eb9