Fix a11y crash when window layer isn't unique.

TalkBack is seeing crashes that I can only explain by our assumption
that window layer is unique in all cases. TalkBack reports that it
happens during animation, so I assume that the layer may repeat
transiently.

Reducing our dependence on this assumption by traversing the list of
windows sorted by layer without assuming that the list has the same
length as the list of unsorted windows.

Also documenting the undefined behavior of SparseArray when indexing
beyond its bounds. The undefined behavior itself is intentional for
performance reasons.

Bug: 28679528
Bug: 28815817
Change-Id: I0c9f90b0b458b4cde465f603ba204fe6691e5c2c