fix(ripple): multiple touches causing ripples to start from center

Fix is pretty simple, we weren't clearing out the first pointerdown event, and subsequent ones were ignored since the `pointerId` changes with each touch.

Fixes #5349

PiperOrigin-RevId: 596047639

ripple/internal/ripple.ts

@@ -374,6 +374,7 @@ export class Ripple extends LitElement implements Attachable {
   }
 
   private async endPressAnimation() {
+    this.rippleStartEvent = undefined;
     this.state = State.INACTIVE;
     const animation = this.growAnimation;
     let pressAnimationPlayState = Infinity;