Wrap Flutter Channel method calls to execute on main thread

[chadpav]

All calls from Native to Flutter must execute on the main UI thread. This PR wraps all calls to the FlutterChannel in DispatchQueue.main.async { ... } to force execution.

See https://docs.flutter.dev/platform-integration/platform-channels#channels-and-platform-threading for more information.

I know this was causing warnings to get printed to the debug console in Flutter. I suspect that could cause intermittent problems at runtime. It's better to address it just in case.

[VERBOSE-2:shell.cc(1004)] The 'sstonn/flutter_watch_os_connectivity_callback' channel sent a message from native to Flutter on a non-platform thread. Platform channel messages must be sent on the platform thread. Failure to do so may result in data loss or crashes, and must be fixed in the plugin or application code creating that channel.

Addresses issue #4

[ChrisElliotUK]

Can we get this merged in and on pub.dev

[ChrisElliotUK]

@ssttonn are you able to merge this?

[MattyBoy4444]

Can this be merged?