Android ActionBar/ActionBarSherlock multiple choices spinner

A Spinner shows the drop down using a ListPopupWindow, you could use the same to show that multi choice item selection list:

private void showPopup() {
    final ListPopupWindow lpw = new ListPopupWindow(this);
    lpw.setAdapter(/*Your adapter here*/);
    lpw.setAnchorView(mAnchor); // see below
    lpw.setContentWidth(/*specific value*/); // see below
    lpw.show();
    // this is required because the popup's `ListView` will not be available 
    // until the ListPopupWindow is actually shown.
    mAnchor.post(new Runnable() {
        @Override
        public void run() {
            lpw.getListView().setChoiceMode(AbsListView.CHOICE_MODE_MULTIPLE);
        }
    });
}

You could then call this method from the onOptionsItemSelected() callback when the right MenuItem is selected. There are two other things you need to take care:

The mAnchor is a View that you need to insert in the Activity's layout in the top-right corner so the ListPopupWindow will show in the right position. For example, if you have as an Activity root:

a RelativeLayout then mAnchor will be:

mAnchor = new View(this);
RelativeLayout.LayoutParams rlp = new RelativeLayout.LayoutParams(0, 0);
rlp.addRule(RelativeLayout.ALIGN_PARENT_RIGHT, RelativeLayout.TRUE);
rlp.addRule(RelativeLayout.ALIGN_PARENT_TOP, RelativeLayout.TRUE);
mAnchor.setLayoutParams(rlp);
// add mAnchorView first to the RelativeLayout

a LinearLayout then mAnchor will be:

mAnchor = new View(this);
LinearLayout.LayoutParams llp = new LinearLayout.LayoutParams(0, 0);
llp.gravity = Gravity.RIGHT;
mAnchor.setLayoutParams(llp);
// add mAnchorView first to the LinearLayout(assuming orientation vertical)

and so on for other types of layouts.

Secondly, you need to setup the width of the ListPopupWindow to a desired value. You'll need to adapt this value for different screen sizes and orientation(like phone-portrait and phone-landscape, different table sizes in portrait and landscape).