Fix leak in LayoutTransition

LayoutTransition was making an incorrect assumption that there could
only be one transition animation on a child of a transitioning container.
But if multiple children are added/removed to/from that container, there would
be multiple calls to set up changing animations for each existing child
of that container. This meant that the child would have multiple, new
OnLayoutChangeListeners added to it as part of the setup process.

Meanwhile, we would cache only the latest listener in a hashmap that used
the child as a key for the listener. Then when we cleaned up the hashmap later,
we would remove only the latest listener from the child, leaving the rest there
for eternity.

The fix is to skip the setup entirely for children that already have listeners
set on them; they must, if that's the case, already have been set up and are
already listening for layout changes. Setting up the animation is redundant,
and adding another listener is a leak.

issue #5588509: memory leak in systemui

Change-Id: I2c9f312cc2bcf4f2d08ac6b5d8f8e495aa4f3597