Expose public API to control mIsChildViewEnabled

This is needed to replace an @UnsupportedApiUsage of the field
in appcompat/widget/DropDownListView.java

Bug: 177008210
Test: AbsListViewTest.testSelectedChildViewEnabled

Change-Id: Ia0bfe3f70f62db8185e1d9784d921e2e1f291006
2 files changed