From http://stackoverflow.com/questions/26671412/android-lollipop-toolbar-switch-between-open-close-drawer-and-back-button/26671641?noredirect=1#comment41948031_26671641

back button on toolbar

    mToolbar.setNavigationOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            if(shouldBack()) {
               //call onbackpressed or something
                if(displayBackAgain)
                    return; //return after so you don't call syncState();       
            }else if (mNavigationDrawerFragment.isDrawerOpen())
                mNavigationDrawerFragment.closeDrawer();
            else
                mNavigationDrawerFragment.openDrawer();
            mNavigationDrawerFragment.getActionBarDrawerToggle().syncState();
        }
    });
}