[flutter_webrtc] Fix constraint parsing crash and buffer deleter

[seungsoo47]

Port two fixes from the upstream common C++ implementation:

• Skip media constraints with unsupported value types instead of reading them as int, which threw std::bad_variant_access (e.g. for null). fix: skip unrecognized constraint value types in ParseConstraints flutter-webrtc/flutter-webrtc#2132
• Hold the RGB frame buffer allocated with new[] in shared_ptr<uint8_t[]> so it is released with delete[]. Using delete[] instead of delete flutter-webrtc/flutter-webrtc#2039