fix(checkbox): disabled property not being coerced

Fixes the `disabled` property not being coerced like all the other components.

Fixes #13739.

Author: crisbeto

src/lib/checkbox/checkbox.ts

@@ -246,7 +246,7 @@ export class MatCheckbox extends _MatCheckboxMixinBase implements ControlValueAc
   get disabled() { return this._disabled; }
   set disabled(value: any) {
     if (value != this.disabled) {
-      this._disabled = value;
+      this._disabled = coerceBooleanProperty(value);
       this._changeDetectorRef.markForCheck();
     }
   }