Android : mark of checkbox disappear on filter in a listview

I am trying to create list of contacts with checkbox in each item of the listview and name of the contact. I have written a filter which searches by name of the contact. The problem is when I filter through some name and then check that item and then delete or remove some text in the filter then the checkbox of the item which I checked before is disabled and basically a new view is getting created.

    public class ContactAdapter extends BaseAdapter implements Filterable{

    Context context;
    ArrayList<ContactItem> contactlist;
    ArrayList<ContactItem> mStringFilterList;
    ValueFilter valueFilter;

    public ContactAdapter(Context context , ArrayList<ContactItem> contactlist) {
        this.context = context;
        this.contactlist = contactlist;
        mStringFilterList = contactlist;
    }

    @Override
    public int getCount() {
        return contactlist.size();
    }

    @Override
    public Object getItem(int position) {
        return contactlist.get(position);
    }

    @Override
    public long getItemId(int position) {
        return contactlist.indexOf(getItem(position));
    }

    private class ViewHolder {
        TextView name;
        CheckBox selection;
        TextView phoneNumber;
    }

    @Override
    public View getView(final int position, View convertView, ViewGroup parent) {
        ViewHolder holder = null;

        if (convertView == null) {
            LayoutInflater mInflater = (LayoutInflater) context
                    .getSystemService(Activity.LAYOUT_INFLATER_SERVICE);
            convertView = mInflater.inflate(R.layout.adapter_item_contact,null);
            holder = new ViewHolder();
            holder.name = (TextView) convertView.findViewById(R.id.tv_name);
            holder.phoneNumber = (TextView) convertView.findViewById(R.id.tv_number);
            holder.selection = (CheckBox) convertView.findViewById(R.id.cb_contact);
            convertView.setTag(holder);

            holder.selection.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View view) {
                    CheckBox cb = (CheckBox) view;
                    ContactItem item = (ContactItem) cb.getTag();
                    item.setSelected(cb.isChecked());

                }
            });
        }else{
            holder = (ViewHolder) convertView.getTag();
        }

        ContactItem contact = contactlist.get(position);
        holder.name.setText(contact.getName());
        holder.phoneNumber.setText(contact.getPhoneNumber());
        holder.selection.setTag(contact);
        holder.selection.setChecked(contact.isSelected());


        return convertView;
    }

    @Override
    public Filter getFilter() {
        if (valueFilter == null) {
            valueFilter = new ValueFilter();
        }
        return valueFilter;
    }

    private class ValueFilter extends Filter {
        @Override
        protected FilterResults performFiltering(CharSequence constraint) {
            FilterResults results = new FilterResults();

            if (constraint != null && constraint.length() > 0) {
                ArrayList<ContactItem> filterList = new ArrayList<ContactItem>();
                for (int i = 0; i < mStringFilterList.size(); i++) {
                    if ( (mStringFilterList.get(i).getName().toUpperCase() )
                            .contains(constraint.toString().toUpperCase())) {

                        ContactItem contact = new ContactItem(mStringFilterList.get(i)
                                .getName() ,  mStringFilterList.get(i)
                                .getPhoneNumber() ,  mStringFilterList.get(i)
                                .isSelected());

                        filterList.add(contact);
                    }
                }
                results.count = filterList.size();
                results.values = filterList;
            } else {
                synchronized(this){
                    results.count = mStringFilterList.size();
                    results.values = mStringFilterList;
                }

            }
            return results;

        }

        @SuppressWarnings("unchecked")
        @Override
        protected void publishResults(CharSequence constraint,
                                      FilterResults results) {
            contactlist = (ArrayList<ContactItem>) results.values;
            notifyDataSetChanged();
        }

    }

}