Created FlutterSurrogateBinaryMessenger to make sure that channels are
holding onto engines and not viewcontrollers. This doesn't change the
public API but makes clients do what we want them to be doing, using
Engine for FlutterBinaryMessenger.
When the macOS framework was initially landed, it included some iOS code
using a localized hack. This reorganizes the code structure to clearly
indicate which files are shared between iOS and macOS, vs. those that
are platform-specific.
