Fix potential off-by-one

Being consistent with the notion of mEndChildIndex, it should be -1 by
default. Otherwise it assumes the display list has a single element.
This is consistent with EndReorderBarrierDrawable constructor that would
set mEndChildIndex to -1 if display list is empty.

Bug: 117921091
Test: all tests should pass
Change-Id: I64b84c789ad8126ac8f0b8c2ebfc128635e5b30d