Fix fab (#458)

* Only destroy CollapsingContentView

* Fix Fab

android/app/src/main/java/com/reactnativenavigation/screens/CollapsingViewPagerScreen.java

     public void destroy() {
         super.destroy();
         for (ContentView contentView : contentViews) {
-            ((CollapsingContentView) contentView).destroy();
+            if (contentView instanceof CollapsingContentView) {
+                ((CollapsingContentView) contentView).destroy();
+            }
         }
     }
 }

src/deprecated/platformSpecificDeprecated.android.js

 }
 
 function getFab(screen) {
-  if (screen.fab) {
-    const fab = screen.fab;
+  if (screen.navigatorButtons && screen.navigatorButtons.fab) {
+    const fab = screen.navigatorButtons.fab;
     const collapsedIconUri = resolveAssetSource(fab.collapsedIcon);
     if (!collapsedIconUri) {
       return;