Sync client_wrapper with upstream Flutter

[seungsoo47]

Apply the upstream client_wrapper changes that the source syncs missed. They are already in the pinned Flutter commit (6655482ec0):

• Take StreamHandlerError strings by const reference. Conform to clang_tidy in client_wrapper headers. flutter-team-archive/engine#46058 (42bac1a9a9)
• Move std::function arguments instead of copying them, and take the MethodChannel handler by const reference. Roll Clang to 8c7a2ce01a77c96028fe2c8566f65c45ad9408d3 flutter/flutter#173429 (f45a8f8b71)
• Point the channel_buffers.dart link to the flutter/flutter repo. Replace References to flutter/engine with flutter/flutter flutter/flutter#182600 (2d7e80963b)

[JSUYA]

#36
In the past, modifications of this type were rolled back due to c++ breaks.
For this reason, there are specific parts we intentionally preserve in the diff during version updates.
You need to verify that no issues arise.

[seungsoo47]

#36 In the past, modifications of this type were rolled back due to c++ breaks. For this reason, there are specific parts we intentionally preserve in the diff during version updates. You need to verify that no issues arise.

Thank you for the review. I checked the diffs we keep on purpose: the .get() patch in event_channel.h (#36) and the removal of engine_method_result.cc (#44). This PR doesn't change either of them. The #36 fix made the StreamHandlerError members values; this PR only takes the constructor parameters by const&, and the members are still values.

Verified:

• Embedder CI passes, and the arm and arm64 builds succeed.
• Plugins that use the changed APIs build with the Tizen SDK LLVM-10 / rootstrap 6.0 toolchain.
• On an RPi4 (Tizen 10.1), with the embedder built from this branch, the integration tests give the same results as before.
  messageport, geolocator and video_player pass.