[Tech Debt] Change `TARGET_OS_IPHONE` macros in code

Xcode 10 and Swift 4.2 no longer recognize the `TARGET_OS_IPHONE` macro, which was deprecated in Swift 3 and removed in 4.2. The side effect of this is that Xcode is now executing code that was not meant for simulators.

The code need reviewed and all `TARGET_OS_IPHONE` macros replaced. I don't have a specific recommendation because this project may also need updated to target iOS 9.2 instead of 9.0 for full compatibility with the new macros.

Ref. https://github.com/Automattic/Automattic-Tracks-iOS/pull/78
Ref. https://github.com/woocommerce/woocommerce-ios/pull/475