common demo: fix loading button never stopping

Because we now use scoped elements and we looked for the static name
instead of the dynamically generated one in the DOM.

packages/common/dbp-common-demo.js

@@ -87,7 +87,8 @@ export class DbpCommonDemo extends ScopedElementsMixin(LitElement) {
 
     buttonClickHandler() {
         setTimeout(() => {
-            this.shadowRoot.querySelector("dbp-button").stop();
+            const scopedTagName = this.constructor.getScopedTagName('dbp-button');
+            this.shadowRoot.querySelector(scopedTagName).stop();
         }, 1000);
     }