Currently, ndk cpu-features is being used.
This is now deprecated and google open-source
cpu_features library is recommended.
PiperOrigin-RevId: 273987647
If a user has checked out libgav1 they almost certainly want to
build the extension, so fail if CMake is not present but
libgav1 is present. Also add a note to the README linking to
instructions for installing CMake.
Also remove the NDK check, as ndk.dir is apparently deprecated
and Android Studio appears to install this automatically anyway.
PiperOrigin-RevId: 273918142
