Use EnterExitTransition internally for animated item fade

Right now, we specify some fade in animation spec and fade out animation
spec for lazy layout animated items. In the future, we want to tspecify
the full range of enter exit transitions using the EnterExitTransition
API. This CL internally wraps the fade in specs in enter and exit
transitions in preparation for using the enter exit api.

This change allows us to refactor the lazy layout item animation code to
start utilising enter and exit internally which allows us to then
integrate the other range of available animations without exposing the
ability to use those animations to the user

Test: N/A
Change-Id: I57e358db1eb6ffbed80c83dc1e512e690c1c13c8