Set ENABLE_NONFREE_KERNELS=ON

Nonfree kernels are used for some video processing and video encoding
tasks. They have the same license as the rest of the code, so we can
still use them in Fuchsia. The reason they're listed as nonfree is that
the blob source code hasn't been made public.

Change-Id: Id228c3bfe589da8a352a2576da527883b6f28c38