Button toggle does not follow material design spec

[BrummbQ]

Bug, feature request, or proposal:

The current button toggle styling differs from the guideline: https://material.io/design/components/buttons.html#toggle-button

i.e.:

• There should be no shadow/elevation, just a border around the items
• the angular component is missing the hover effect

Is there a reason for these differences and would you accept pull requests?

[crisbeto]

We're in the process of aligning all the components with the latest iteration of the Material design spec. Here's the PR for the button toggle in particular. #12443

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