Android: Support unified headers in NDK r14

Google is unifying all their platform headers as described in this document: https://android.googlesource.com/platform/ndk/+/master/docs/UnifiedHeaders.md

It would be interesting if CMake could support that as an option. As per the roadmap, they will default to having them disabled by default in r14, but probably enabled by default in r15. I can try to have a look at the issue if you don't have time before 3.8, but you're probably a bit more familiar with the code than me still :)