Just change the code
hsv_channel.get(0).setTo(new Scalar(145,25,45));
to
hsv_channel.get(0).setTo(new Scalar(145)); // whatever your value
hsv_channel.get(0) is single channel Mat so your Scalar value should be with single.
Edit:-
Yo can see the OpenCV camera preview example here
On Tutorial1Activity.java you can see a method
public Mat onCameraFrame(CvCameraViewFrame inputFrame)
Add these lines on this method
src = inputFrame.rgba();
Imgproc.cvtColor(src, hsv,Imgproc.COLOR_RGB2HSV );
Core.split(hsv, hsv_channel);
// Imgproc.equalizeHist(hsv_channel.get(0), hsv_channel.get(0));
hsv_channel.get(0).setTo(new Scalar(64));
Core.merge(hsv_channel, hsv);
Imgproc.cvtColor(hsv, src,Imgproc.COLOR_HSV2RGB );
//return hsv_channel.get(0);
return src;