Fix edge case for ItemGroup notify propagation

When removing the last item in a nested ItemGroup, make sure
getChildPosition returns the position of where the item would have
been so that the removal notification can be propagated properly.

Test: ./gradlew connectedAndroidTest test
Bug: 37361727
Change-Id: I50f5ceb50c2c7432056427a9d11a181dc384f4e7