[api-minor] Further modernize the ProgressBar class (PR 14918 follow-up)

- Simplify how we look-up the DOM-element, which should also be a tiny bit more efficent.

 - Use private class-fields, rather than property-names prefixed with underscores.

 - Inline the `#updateBar` helper-method directly in the `percent`-setter, since having a separate method doesn't seem necessary in this case.

 - Set the `indeterminate`-class on the ProgressBar DOM-element, to simplify the code.

Finally, also (slightly) re-factors the `PDFViewerApplication.progress`-method to make it a bit smaller.

examples/mobile-viewer/viewer.css

@@ -221,13 +221,13 @@ canvas {
   }
 }
 
-#loadingBar .progress.indeterminate {
+#loadingBar.indeterminate .progress {
   transform: none;
   background-color: rgba(153, 153, 153, 1);
   transition: none;
 }
 
-#loadingBar .indeterminate .glimmer {
+#loadingBar.indeterminate .progress .glimmer {
   position: absolute;
   top: 0;
   left: 0;

examples/mobile-viewer/viewer.js

@@ -161,7 +161,7 @@ const PDFViewerApplication = {
   },
 
   get loadingBar() {
-    const bar = new pdfjsViewer.ProgressBar("#loadingBar");
+    const bar = new pdfjsViewer.ProgressBar("loadingBar");
 
     return pdfjsLib.shadow(this, "loadingBar", bar);
   },