Summary:
This diff moves the iOS descriptors and other layout helpers into its own buck target. Earlier everything was part of the FlipperKitLayoutPlugin target. When kcui, tried to add macos descriptors, we realised that FlipperKitLayoutPlugin doesn't have macos platform in it and thus he had to put #if TARGET_OS_SIMULATOR check in iOS descriptors and at some other places.
In order to make the integration easier I decided to split the Layout Plugin in following way.
```
FlipperKitLayoutPlugin
-> FlipperKitLayoutIOSDescriptors (platform set as IOS)
-> FlipperKitLayoutHelpers (used in iOS Descriptors and usefule for Macos too)
-> FlipperKitLayoutMacOSDescriptors(coming soon by kcui)(platform set as MacOS)
```
Reviewed By: kcui
Differential Revision: D27127587
fbshipit-source-id: f108ef81914297e93c886fe53dabf0a4bf11ebc1
