Add TabLayout.getItemCount method

Related to https://github.com/NativeScript/NativeScript/issues/3147

android/widgets/src/main/java/org/nativescript/widgets/TabLayout.java

@@ -210,6 +210,13 @@ public class TabLayout extends HorizontalScrollView {
         return result;
     }
 
+    /**
+     * Gets the number of realized tabs.
+     */
+    public int getItemCount(){
+        return this.mTabStrip.getChildCount();
+    }
+
     /**
      * Create a default view to be used for tabs.
      */