fix(material/chips): make focus overlay not affect text color

Make focus overlay not affect the text color.

Give the text label a higher z-index than the focus overlay to ensure
that it does not affect the text color. Material spec requires that
state layer does not interfere with text color.

Fix issue where focus overlay would be on top of the text and darken the color
of the text. This is a deviation from spec and causes a11y issues due to
insufficient text contrast.

Author: zarend

src/material/chips/chip.scss

@@ -253,6 +253,11 @@
     $offset: calc(#{$border-width} + 2px);
     margin: calc(#{$offset} * -1);
   }
+
+  // Give the text label a higher z-index than the focus overlay to ensure that the focus overlay
+  // does not affect the color of the text. Material spec requires state layer to not interfere with
+  // text color.
+  z-index: 1;
 }
 
 .mat-mdc-chip-remove {