Override BottomTabs props only if props are undefined

closes #312

src/deprecated/platformSpecificDeprecated.android.js

     addNavigatorButtons(tab, params.drawer);
     addNavigationStyleParams(tab);
     addTabIcon(tab);
-    tab.passProps = params.passProps;
+    if (!tab.passProps) {
+      tab.passProps = params.passProps;
+    }
 
     adaptTopTabs(tab, tab.navigatorID);