Disable layers prioritization code in extensions (ampproject#21445)

These extensions added layers support with the understanding that layers
redoes the prioritization of elements. Now that we've experiment guarded
the prioritization code, we need to guard here too.

extensions/amp-carousel/0.1/scrollable-carousel.js

@@ -64,7 +64,8 @@ export class AmpScrollableCarousel extends BaseCarousel {
     this.container_.classList.add('i-amphtml-scrollable-carousel-container');
     this.element.appendChild(this.container_);
 
-    this.useLayers_ = isExperimentOn(this.win, 'layers');
+    this.useLayers_ = isExperimentOn(this.win, 'layers') && isExperimentOn(
+        this.win, 'layers-prioritization');
 
     this.cells_.forEach(cell => {
       if (!this.useLayers_) {

extensions/amp-image-slider/0.1/amp-image-slider.js

@@ -138,7 +138,8 @@ export class AmpImageSlider extends AMP.BaseElement {
         '2 <amp-img>s must be provided for comparison');
 
     // TODO(kqian): remove this after layer launch
-    if (!isExperimentOn(this.win, 'layers')) {
+    if (!isExperimentOn(this.win, 'layers') ||
+        !isExperimentOn(this.win, 'layers-prioritization')) {
       // see comment in layoutCallback
       // When layers not enabled
       this.setAsOwner(dev().assertElement(this.leftAmpImage_));