fix(icon): icons should have a minimum height.

* Icons should not shrink on smaller viewport. (At both axis, looks weird and is inconsistent)
* Also fixes an issue in the toolbar demo, where the icons shrink, when the viewport is to small.

Fixes #7599

Closes #7860

Author: devversion

src/components/icon/icon.scss

@@ -7,6 +7,10 @@ md-icon {
   height: $icon-size;
   width: $icon-size;
 
+  // The icons should not shrink on smaller viewports.
+  min-height: $icon-size;
+  min-width: $icon-size;
+
   svg {
     pointer-events: none;
     display: block;