Have focus avoid middle of a foucs chain when possible.

Bug 24873983

Focus follows top->bottom left->right by default (LTR),
but that could put next focus on a View in the middle
of a focus chain (something else points to the View with
nextFocusForwardId). That means that the View pointing
to it may get skipped or it may end in a focus loop
in the chain.

Instead, we now prefer to have the next view be one that
is at the head of a chain (forward) or tail of a chain
(backward).

Change-Id: Icb59f01c1406bc89ef3494fdc5be109e465aa8bc