client_wrapper: merge common/client_wrapper

[HidenoriMatsubayashi]

Merged the latest source files from flutter/engine repo.

• src/flutter/shell/platform/common/client_wrapper/include/flutter/encodable_value.h => https://github.com/flutter/engine/commit/a3c5a319fccb7b137949ee7f410bf2510148dd3d

• src/flutter/shell/platform/common/client_wrapper/include/flutter/event_channel.h src/flutter/shell/platform/common/client_wrapper/include/flutter/event_stream_handler.h => https://github.com/flutter/engine/commit/52574efd2a88390575423b6407ffd13a39f58429

• src/flutter/shell/platform/common/client_wrapper/include/flutter/method_channel.h => https://github.com/flutter/engine/commit/4165d4b9b6f1843e91ccbf13a4c12a32d48ca648

• src/flutter/shell/platform/common/client_wrapper/include/flutter/plugin_registrar.h => https://github.com/flutter/engine/commit/6d263ea56a62a9cec738f5cfa6c1afcacfd14e9d

• src/flutter/shell/platform/common/client_wrapper/standard_codec.cc => https://github.com/flutter/engine/commit/71ee5f19bc164c140055007d39bb3240d04a2376