perf(infinitescroll): display none svg until needed

By default the SVG spinner should be display none at all times. Only
display block the SVG when the infinite scroll is in the loading state.

Closes #5776

ionic/components/infinite-scroll/infinite-scroll.scss

@@ -22,13 +22,14 @@ ion-infinite-scroll-content {
   flex-direction: column;
   justify-content: center;
   height: 100%;
+  min-height: 84px;
   text-align: center;
 }
 
 .infinite-loading {
+  display: none;
   margin: $infinite-scroll-loading-margin;
   width: 100%;
-  opacity: 0;
 }
 
 .infinite-loading-text {
@@ -41,7 +42,7 @@ ion-infinite-scroll-content {
 // --------------------------------------------------
 
 ion-infinite-scroll-content[state=loading] .infinite-loading {
-  opacity: 1;
+  display: block;
 }
 
 ion-infinite-scroll-content[state=disabled] {