fix(checkbox): incorrect ripple color when unchecked

Currently no matter whether the checkbox is checked, its ripple always uses the theme color. These changes align it with the other components and the spec where the ripple is the base color when unchecked.

Author: crisbeto

src/lib/checkbox/_checkbox-theme.scss

@@ -90,17 +90,24 @@
     }
   }
 
-  .mat-checkbox:not(.mat-checkbox-disabled) {
-    &.mat-primary .mat-checkbox-ripple .mat-ripple-element {
-      background-color: mat-color($primary);
+  // Switch this to a solid color since we're using `opacity`
+  // to control how opaque the ripple should be.
+  .mat-checkbox .mat-ripple-element {
+    background-color: map_get(map-get($theme, foreground), base);
+  }
+
+  .mat-checkbox-checked:not(.mat-checkbox-disabled),
+  .mat-checkbox:active:not(.mat-checkbox-disabled) {
+    &.mat-primary .mat-ripple-element {
+      background: mat-color($primary);
     }
 
-    &.mat-accent .mat-checkbox-ripple .mat-ripple-element {
-      background-color: mat-color($accent);
+    &.mat-accent .mat-ripple-element {
+      background: mat-color($accent);
     }
 
-    &.mat-warn .mat-checkbox-ripple .mat-ripple-element {
-      background-color: mat-color($warn);
+    &.mat-warn .mat-ripple-element {
+      background: mat-color($warn);
     }
   }
 }