code: public void setHomeAsUpIndicator(Drawable indicator) {
          if (indicator == null) {
              mHomeAsUpIndicator = getThemeUpIndicator();
              mHasCustomUpIndicator = false;
          } else {
              mHomeAsUpIndicator = indicator;
              mHasCustomUpIndicator = true;
          }
          if (!mDrawerIndicatorEnabled) {
              setActionBarUpIndicator(mHomeAsUpIndicator, 0);
          }
      }
docs: Set the up indicator to display when the drawer indicator is not
  enabled.
  <p/>
  If you pass <code>null</code> to this method, the default drawable from
  the theme will be used.
  
  @param indicator A drawable to use for the up indicator, or null to use
  the theme's default
  @see #setDrawerIndicatorEnabled(boolean)