Only modify add Transition targets when they aren't targeted.

Bug 18191727

Transitions were targeting specific views, but they do this by
adding targets. If there was already a view targeted, now that
transition also targeted a different view. This prevents that
by only targeting views when no target was given.

Change-Id: I50590a81c1c1c55b1624f8df848ab53731f7e8e1