Fixes button partially showing when parent view height is 0 (facebook#26435)

Summary:
When the parent view that wraps a button has height 0, the button is still shown partially because of the padding given for text inside Button component for iOS. Here is the issue raised for that:
facebook#26421

Probably, we should not hard code these values, rather provide a way to provide custom style ? This is my first PR so not making big change. :D

## Changelog

[iOS] [Fixed] - Give margin instead of padding to text in Button component
Pull Request resolved: facebook#26435

Test Plan:
When using this block of code,
```
<View style={{height:0}}>
    <Button title="There is an issue"></Button>
</View>
```
Before:
<img width="284" alt="image" src="https://user-images.githubusercontent.com/5866078/64905271-6c129700-d6f5-11e9-86c1-c301eb8123f3.png">

After:
<img width="283" alt="image" src="https://user-images.githubusercontent.com/5866078/64905284-8cdaec80-d6f5-11e9-9589-28d8d01c8ba1.png">

Differential Revision: D17661181

Pulled By: cpojer

fbshipit-source-id: 62b04123d9edb4d760bd54d96ae0615c1ccff7ab

Author: roshangm1

Libraries/Components/Button.js

@@ -204,7 +204,7 @@ const styles = StyleSheet.create({
   }),
   text: {
     textAlign: 'center',
-    padding: 8,
+    margin: 8,
     ...Platform.select({
       ios: {
         // iOS blue from https://developer.apple.com/ios/human-interface-guidelines/visual-design/color/