Previously, FlutterEngine.binaryMessenger returned the FlutterEngine
instance itself, which meant that channels/plugins could hold a strong
reference to the engine and thus cause a retain cycle, preventing the
engine from being deallocated.
We introduce FlutterBinaryMessengerRelay, which implements the
FlutterBinaryMessenger protocol, by delegating back to the engine, to
which it holds a weak reference, thus avoiding the retain cycle.
This also changes the FlutterBinaryMessengerRelay.parent property from
an assign property to a weak property since that code is compiled with
ARC enabled as of https://github.com/flutter/engine/pull/44395.
This patch also rearranges the headers to comply with our style guide:
related headers for this file, C headers, C++ headers, other Flutter
headers, in that order.
Fixes: https://github.com/flutter/flutter/issues/116445
e43adf1acd