Measure TopBar buttons using using MeasureSpec.UNSPECIFIED

This change lets buttons stretch horizontally using padding and
margins and not only by changing width property explicitly.

lib/android/app/src/main/java/com/reactnativenavigation/views/titlebar/TitleBarReactButtonView.java

 
     @Override
     protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
-        super.onMeasure(
-                getChildCount() > 0 ? MeasureSpec.makeMeasureSpec(getChildAt(0).getWidth(), MeasureSpec.EXACTLY) : widthMeasureSpec,
-                heightMeasureSpec
-        );
+        super.onMeasure(MeasureSpec.makeMeasureSpec(MeasureSpec.getSize(widthMeasureSpec), MeasureSpec.UNSPECIFIED), heightMeasureSpec);
     }
 }

playground/src/screens/CustomRoundedButton.js

 
   render() {
     return (
-      <View style={styles.container} key={'guyguy'}>
+      <View style={styles.container}>
         <View style={styles.button}>
           <TouchableOpacity onPress={() => Alert.alert(this.props.title, 'Thanks for that :)')}>
             <Text style={styles.text}>{this.props.title}</Text>