EnhancedMenuInflater.java

import android.support.v4.internal.view.SupportMenuItem;
import android.support.v7.internal.view.menu.MenuItemImpl;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;

import here.is.your.R;

public class EnhancedMenuInflater {

    public static void inflate(MenuInflater inflater, Menu menu, boolean forceVisible) {
        inflater.inflate(R.menu.menu, menu);

        if (!forceVisible) {
            return;
        }

        int size = menu.size();
        for (int i = 0; i < size; i++) {
            MenuItem item = menu.getItem(i);
            // check if app:showAsAction = "ifRoom"
            if (((MenuItemImpl) item).requestsActionButton()) {
                item.setShowAsAction(SupportMenuItem.SHOW_AS_ACTION_ALWAYS);
            }
        }
    }
}

layout

<here.is.my.SplitToolbar
	android:id="@+id/toolbar"
	android:layout_width="match_parent"
	android:layout_height="wrap_content"
	android:layout_alignParentBottom="true"/>

MainActivity.java

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    if (toolbar == null) {
        EnhancedMenuInflater.inflate(getMenuInflater(), menu, false);
    }
    return super.onCreateOptionsMenu(menu);
}

// somewhere after views have been set.
if (toolbar != null) {
    EnhancedMenuInflater.inflate(getMenuInflater(), toolbar.getMenu(), true);
    toolbar.setOnMenuItemClickListener(new Toolbar.OnMenuItemClickListener() {
        @Override
        public boolean onMenuItemClick(MenuItem item) {
            return onOptionsItemSelected(item);
        }
    });
}

SplitToolbar.java

import android.content.Context;
import android.support.v7.widget.ActionMenuView;
import android.support.v7.widget.Toolbar;
import android.util.AttributeSet;
import android.view.View;
import android.view.ViewGroup;

public class SplitToolbar extends Toolbar {
    public SplitToolbar(Context context) {
        super(context);
    }

    public SplitToolbar(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public SplitToolbar(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public void addView(View child, ViewGroup.LayoutParams params) {
        if (child instanceof ActionMenuView) {
            params.width = LayoutParams.MATCH_PARENT;
        }
        super.addView(child, params);
    }
}