Fix leak in LayoutTransition

Static objects were referencing the first LayoutTransition object,
which referenced its target container, which eventually referenced the
Activity. That reference chain never went away.
The fix is to create animators that do not reference any target
object (they are just templates which are loaded with real target
objects when they are started).

Change-Id: Ie29f53bf3b886b8052b6ee3a56647a312d9adeaa