Minor contextualMenu fixes (#601)

* Pass showAsAction property to native. It was left out by mistake
* Support textual icons in contextual menu

android/app/src/main/java/com/reactnativenavigation/params/parsers/ContextualMenuButtonParamsParser.java

 
     private ContextualMenuButtonParams parseSingleContextualMenuButton(Bundle button) {
         ContextualMenuButtonParams result = new ContextualMenuButtonParams();
-        result.icon = ImageLoader.loadImage(button.getString("icon"));
+        if (button.get("icon") != null) {
+            result.icon = ImageLoader.loadImage(button.getString("icon"));
+        }
         result.showAsAction = parseShowAsAction(button.getString("showAsAction"));
         result.color = StyleParams.Color.parse(button, "color");
         result.label = button.getString("label");

src/deprecated/platformSpecificDeprecated.android.js

 
   params.rightButtons.forEach((button, index) => {
     const btn = {
-      icon: resolveAssetSource(button.icon).uri,
+      icon: resolveAssetSource(button.icon),
+      showAsAction: button.showAsAction,
       color: processColor(button.color),
       label: button.title,
       index
     };
+    if (btn.icon) {
+      btn.icon = btn.icon.uri;
+    }
     contextualMenu.buttons.push(btn);
   });