Fix child view ordering for accessibility.

When reporting views to accessibility services we are ordering the
children in a parent based on their location on the screen. The
initial implementation worked pretty well in practive but violated
the transitivity property leading to rare crashes in apps. The current
implementation does not violate transitivity but does not produce
good ordering.

Given the lack of time and to minumize risk this change uses the
old strategy which works most of the time and if that fails we
fall back to the current strategy. Coming up with a correct strategy
that produces good results requires more time.

bug:17887986

Change-Id: I1c233ecdf318befc315e793696ac48bd6c652ab6