fix(radio): change radio button trigger element to input element (#2838)

Author: tinayuangao

src/lib/radio/radio.html

@@ -6,7 +6,7 @@
     <div class="md-radio-outer-circle"></div>
     <div class="md-radio-inner-circle"></div>
     <div md-ripple *ngIf="!_isRippleDisabled()" class="md-radio-ripple"
-         [mdRippleTrigger]="_getHostElement()"
+         [mdRippleTrigger]="_getInputElement()"
          [mdRippleCentered]="true"
          [mdRippleSpeedFactor]="0.3"
          mdRippleBackgroundColor="rgba(0, 0, 0, 0)"></div>

src/lib/radio/radio.ts

@@ -500,8 +500,8 @@ export class MdRadioButton implements OnInit {
     }
   }
 
-  _getHostElement() {
-    return this._elementRef.nativeElement;
+  _getInputElement() {
+    return this._inputElement.nativeElement;
   }
 }