Prevent texture removal after TextureManager destroyed

On shutdown, if there is an ImageView actor with an
AnimatedImageVisual still in the tree, then on destruction it tries to
kill off textures. However, TextureManager has already been shutdown
and can't be called.

Adding a lifecycle observer so AnimatedImageVisual helper classes can shutdown cleanly.

Change-Id: I5f07288291b850e69f45df879cce8c27bccb3f6e