Relands https://github.com/flutter/engine/pull/20399

Makes BinaryMessenger available from FlutterEngine, rather than just the plugin registrar. This allows for method channels directly in applications without building them as plugins, and matches the other platforms.

Requires some restructuring of code and GN targets in the client wrappers to make the internals in the shared section usable by the implementations of platform-specific parts of the wrappers. Also fixes a latent issue with EnableInputBlocking symbols being declared but not defined for Windows that came up during testing of the restructing.

Fixes https://github.com/flutter/flutter/issues/62871