fix type cast exception in getConstraints()

[mrcsh]

Hello, while trying to see if I can workaround the apparent volume change on windows 11 chrome when muting and unmuting the local participant, I tried to use the Helper.setVolume() which resulting in this error. The attached code avoids the exception and I think is doing the right thing, but the whole js_interop stuff is pretty new.

ERROR 10:04:07.057:<fn>/735:zi9i-7eh8-zqs1-yfpz/4cpsr-7ockd-b9dk1:TypeError: Instance of 'LegacyJavaScriptObject': type 'LegacyJavaScriptObject' is not a subtype of type 'Map<String, dynamic>'
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 297:3  throw_
dart-sdk/lib/_internal/js_shared/lib/rti.dart 1385:3                         _failedAsCheck
dart-sdk/lib/_internal/js_shared/lib/rti.dart 1363:3                         _generalAsCheckImplementation
packages/dart_webrtc/src/media_stream_track_impl.dart 40:38                  getConstraints
packages/core/bloc/livekit.dart 729:44                                       <fn>
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 45:50           <fn>
dart-sdk/lib/async/zone.dart 1407:47                                         _rootRunUnary
dart-sdk/lib/async/zone.dart 1308:19                                         runUnary
dart-sdk/lib/async/future_impl.dart 162:18                                   handleValue
dart-sdk/lib/async/future_impl.dart 838:44                                   handleValueCallback
dart-sdk/lib/async/future_impl.dart 867:13                                   _propagateToListeners
dart-sdk/lib/async/future_impl.dart 643:5                                    [_completeWithValue]
dart-sdk/lib/async/future_impl.dart 713:7                                    <fn>
dart-sdk/lib/async/zone.dart 1399:13                                         _rootRun
dart-sdk/lib/async/zone.dart 1301:19                                         run
dart-sdk/lib/async/zone.dart 1209:7                                          runGuarded
dart-sdk/lib/async/zone.dart 1249:23                                         callback
dart-sdk/lib/async/schedule_microtask.dart 40:11                             _microtaskLoop
dart-sdk/lib/async/schedule_microtask.dart 49:5                              _startMicrotaskLoop
dart-sdk/lib/_internal/js_dev_runtime/patch/async_patch.dart 181:7           <fn>