fix(input): add back pointer-events none removed in angular#3878 (angular#4206)

Author: mmalerba

src/lib/input/input-container.scss

@@ -132,6 +132,7 @@ $mat-input-underline-disabled-background-image:
   top: 0;
 
   font-size: 100%;
+  pointer-events: none;  // We shouldn't catch mouse events (let them through).
   z-index: 1;
   padding-top: 1em;
 
@@ -180,6 +181,7 @@ $mat-input-underline-disabled-background-image:
   width: 100%;
   padding-top: 1em;
   overflow: hidden;
+  pointer-events: none;  // We shouldn't catch mouse events (let them through).
   transform: translate3d(0, 0, 0); // Prevents the label from shifting after the animation is done.
 
   // Keeps the element height since the placeholder text is `position: absolute`.