Keep buttons from changing size when they change state

Right now if a button changes from a light color to a dark color
based on some user action or state change, the button width also
changes, potentially reflowing the content and making the button
move to a completely different location. Both the height and width
need to stay the same (as was discussed in mw-ui standardization
meetings). The effect on the hover-shadow gimmick is virtually
imperceptable.

Bug: 73381
Change-Id: I27edeb4f1317c9d163bc87f54f35a161cdcd686e
Merge "Keep buttons from changing size when they change state"