first item center aligns in SnapHelper in RecyclerView

Here's a simpler version of the accepted answer, that is also more flexible since it doesn't refer to the screen width:

class BoundsOffsetDecoration : ItemDecoration() {
    override fun getItemOffsets(outRect: Rect,
                                view: View,
                                parent: RecyclerView,
                                state: RecyclerView.State) {
        super.getItemOffsets(outRect, view, parent, state)

        val itemPosition = parent.getChildAdapterPosition(view)

        // It is crucial to refer to layoutParams.width (view.width is 0 at this time)!
        val itemWidth = view.layoutParams.width
        val offset = (parent.width - itemWidth) / 2

        if (itemPosition == 0) {
            outRect.left = offset
        } else if (itemPosition == state.itemCount - 1) {
            outRect.right = offset
        }
    }
}

I've written a Medium post describing a step-by-step implementation of this kind of carousels using RecyclerView and SnapHelper, if you need more details.

You can use ItemDecoration, below codes work for the first and last item and also support margin.

import android.content.Context;
import android.graphics.Point;
import android.graphics.Rect;
import android.support.v7.widget.RecyclerView;
import android.view.Display;
import android.view.View;
import android.view.ViewGroup;
import android.view.WindowManager;

public class OffsetItemDecoration extends RecyclerView.ItemDecoration {

    private Context ctx;

    public OffsetItemDecoration(Context ctx) {

        this.ctx = ctx;
    }

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {

        super.getItemOffsets(outRect, view, parent, state);
        int offset = (int) (getScreenWidth() / (float) (2)) - view.getLayoutParams().width / 2;
        ViewGroup.MarginLayoutParams lp = (ViewGroup.MarginLayoutParams) view.getLayoutParams();
        if (parent.getChildAdapterPosition(view) == 0) {
            ((ViewGroup.MarginLayoutParams) view.getLayoutParams()).leftMargin = 0;
            setupOutRect(outRect, offset, true);
        } else if (parent.getChildAdapterPosition(view) == state.getItemCount() - 1) {
            ((ViewGroup.MarginLayoutParams) view.getLayoutParams()).rightMargin = 0;
            setupOutRect(outRect, offset, false);
        }
    }

    private void setupOutRect(Rect rect, int offset, boolean start) {

        if (start) {
            rect.left = offset;
        } else {
            rect.right = offset;
        }
    }

    private int getScreenWidth() {

        WindowManager wm = (WindowManager) ctx.getSystemService(Context.WINDOW_SERVICE);
        Display display = wm.getDefaultDisplay();
        Point size = new Point();
        display.getSize(size);
        return size.x;
    }
}