This replaces all the uses of the ANDROID_NDK define with ANDROID. This is a
preparatory step to remove our custom android toolchain file and rely on the
standard android NDK one instead. Our toolchain file defined both ANDROID and
ANDROID_NDK, while the NDK one just defines ANDROID. So, make things
consistent and just use ANDROID everywhere.
I haven't yet removed the cmake variable with the same name, as we will need to
do something completely different there -- NDK toolchain defines
CMAKE_SYSTEM_NAME to Android, while our current one pretends it's linux.