style.xml中放入<item name="android:listDivider">@drawable/shape</item>，会覆盖默认风格，

DividertemDecoration类中的：

51行final TypedArray a = context.obtainStyledAttributes(ATTRS);会调用系统的分格属性

public final TypedArray obtainStyledAttributes(@StyleableRes int[] attrs) {
   return getTheme().obtainStyledAttributes(attrs);
}