Add @TestApi PopupMenu.getMenuListView

This method returns the ListView representing the menu.
It is required to test the actual contents of a PopupMenu.

Bug: 34076597
Test: android.widget.cts.ToolbarTest#testItemViewAttributes,
          android.widget.cts.PopupMenuTest#testItemViewAttributes

Change-Id: Id72a8f482048d724d4fde15b126e0d08e1d18390