Set CollapsingTitleBar background only once

This is a quick hack to address issue where the expended color was
set after pop even if screen was collapsed.

android/app/src/main/java/com/reactnativenavigation/views/collapsingToolbar/CollapsingTitleBar.java

 
     @Override
     protected void setBackground(StyleParams params) {
-        titleBarBackground = createBackground(params,
-                params.collapsingTopBarParams.expendedTitleBarColor,
-                params.collapsingTopBarParams.scrimColor);
-        setBackground(titleBarBackground);
+        if (titleBarBackground == null) {
+            titleBarBackground = createBackground(params,
+                    params.collapsingTopBarParams.expendedTitleBarColor,
+                    params.collapsingTopBarParams.scrimColor);
+            setBackground(titleBarBackground);
+        }
     }
 
     private TitleBarBackground createBackground(StyleParams styleParams, StyleParams.Color expendedColor, StyleParams.Color collapsedColor) {