Fixes to app-grid demo for Firefox and Edge

[gdevacc12]

Fix #528
https://github.com/PolymerElements/app-layout/issues/528

Images fail to appear in Firefox as --app-grid-item-height is not set. Added to :host section as it only appears in the media query section.

Also added screen and to the media query as Edge seemed to ignore the query otherwise.

Re-tested the demo with Chrome 62, Opera 49 & 48, Firefox 57 & 56 Edge 40.15063.674.0.

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