Summary:
Nested subobjects work but aren't very well typed in the layout plugin.
So changing it to help flipper work with it.
From:
```
positionOnScreen: {x: 234, y: 456},
```
to
```
positionOnScreenX: 234,
positionOnScreenY: 456,
```
Reviewed By: passy
Differential Revision: D19998161
fbshipit-source-id: 4e4d7ced3cb37c527bbdf65549ec436311a8c2b8