fix for runtime exception on Android

[anthonyhunter]

great plugin, thanks

getting runtime exception on Android Methods marked with @UiThread must be executed on the main thread

see Jumping to the UI thread in Android

changes to FlutterBeaconScanner.java to fix

import android.os.Looper;
import android.os.Handler;

private Handler handler;

class FlutterBeaconScanner {

  public FlutterBeaconScanner(...) {
    ...
    handler = new Handler(Looper.getMainLooper());
  }

  private final RangeNotifier rangeNotifier = new RangeNotifier() {
    @Override
    public void didRangeBeaconsInRegion(Collection<Beacon> collection, Region region) {
      if (eventSinkRanging != null) {
        final Map<String, Object> map = ...
        ...
        handler.post(new Runnable(){
          @Override
          public void run() {
            eventSinkRanging.success(map);
          }
        });
      }
    }
  };

  public void didEnterRegion(Region region) {...}
  public void didExitRegion(Region region) {...}
  public void didDetermineStateForRegion(int state, Region region) {...}

}

[supermuesli]

can you fork and add a pr with your changes?

[eleumasc]

Hi, is it possible to release a version that fixes this bug on pub.dev? Thank you in advance!