The `flutter.buildMode` property now controls whether to build in debug,
profiling, or release.

The hello_services example can now be built both for Android and iOS.

This patch adds some sensible defaults to xcode_backend.sh and documents how to
build hello_services for iOS using Xcode.

Currently this service just returns mock data, but it exercises the plumbing.

This example shows how to build a basic app using a custom Xcode project.
Future patches will bring the functionality up to par with hello_android.