Fix null pointer when mColorFadeEnabled is false

mColorFadeOffAnimator and mColorFadeOnAnimator won't ever
be initialized if mColorFadeEnabled is set to false.
This means that we can't assume that these animators always exist.

Test: Manual, lock, unlock device
Change-Id: I0cb7a10f7d311bc992c4a3e1b4943b454e39ec1a
Fixes: 64726143