Add Android support for rootBackgroundImageName (#3051)

android/app/src/main/java/com/reactnativenavigation/controllers/NavigationActivity.java

         if (hasBackgroundColor()) {
             layout.asView().setBackgroundColor(AppStyle.appStyle.screenBackgroundColor.getColor());
         }
+        String rootBackgroundImageName = activityParams.screenParams.styleParams.rootBackgroundImageName;
+        if (rootBackgroundImageName != null) {
+            layout.asView().setBackgroundResource(this.getResources().getIdentifier(rootBackgroundImageName, "drawable", this.getPackageName()));
+        }
         setContentView(layout.asView());
     }
 

android/app/src/main/java/com/reactnativenavigation/params/StyleParams.java

     public int topTabsHeight;
 
     public Color screenBackgroundColor;
+    public String rootBackgroundImageName;
 
     public boolean drawScreenAboveBottomTabs;
 

docs/styling-the-navigator.md

   screenBackgroundColor: 'white', // Default screen color, visible before the actual react view is rendered
   orientation: 'portrait' // Sets a specific orientation to a modal and all screens pushed to it. Default: 'auto'. Supported values: 'auto', 'landscape', 'portrait'
   disabledButtonColor: '#ff0000' // chnaged the navigation bar button text color when disabled.
+  rootBackgroundImageName: 'iOS: <name of image in Images.xcassets>. Android: <name of drawable>', // Static while you transition between screens. Works best with screenBackgroundColor: 'transparent'
   
   // iOS only
   statusBarTextColorSchemeSingleScreen: 'light', // same as statusBarTextColorScheme but does NOT remember across pushes
   disabledBackGesture: false, // default: false. Disable the back gesture (swipe gesture) in order to pop the top screen. 
   disabledSimultaneousGesture: true, // default: true. Disable simultaneous gesture recognition.
   screenBackgroundImageName: '<name of image in Images.xcassets>', // Optional. default screen background image.
-  rootBackgroundImageName: '<name of image in Images.xcassets>', // Static while you transition between screens. Works best with screenBackgroundColor: 'transparent'
-
+  
   navBarButtonFontSize: 20, // Change font size nav bar buttons (eg. the back button) (remembered across pushes)
   navBarButtonFontWeight: '500', // Change font weight nav bar buttons (eg. the back button) (remembered across pushes)
 

src/deprecated/platformSpecificDeprecated.android.js

     topTabsScrollable: originalStyleObject.topTabsScrollable,
     topTabsHeight: originalStyleObject.topTabsHeight,
     screenBackgroundColor: processColor(originalStyleObject.screenBackgroundColor),
+    rootBackgroundImageName: originalStyleObject.rootBackgroundImageName,
 
     drawScreenAboveBottomTabs: !originalStyleObject.drawUnderTabBar,