Override BottomTabs props only if props are undefined

closes #312

src/platformSpecific.android.js

 
   if (params.tabs) {
     _.forEach(params.tabs, (tab) => {
-      tab.passProps = params.passProps;
+      if (!tab.passProps) {
+        tab.passProps = params.passProps;
+      }
       savePassProps(tab);
     });
   }