fix(select): Make CSS-only background transparent (#1499)

Author: roncapat

packages/mdc-select/mdc-select.scss

@@ -43,6 +43,7 @@
   border: none;
   border-bottom: 1px solid rgba(black, .12);
   border-radius: 0;
+  background-color: transparent;
   background-repeat: no-repeat;
   background-position: right center;
   font-family: Roboto, sans-serif;