ExpandableListView 对groupIndicator的解释，但state list中没提到
public void setGroupIndicator (Drawable groupIndicator)
Since: API Level 1

Sets the indicator to be drawn next to a group.
Parameters
groupIndicator The drawable to be used as an indicator. If the group is empty, the state state_empty will be set. If the group is expanded, the state state_expanded will be set.