NDK r16b has unified headers that buck does not support

Created by: sheldonneuberger

In NDK 16, the sysroot is now /usr/local/share/android-sdk/ndk-bundle/sysroot/usr/include instead of /usr/local/share/android-ndk-r13b/platforms/android-19/arch-arm/usr/include. There are a few other changes that need to be made during compilation as well (noted in the UnifiedHeaders doc here https://android.googlesource.com/platform/ndk/+/ndk-release-r16/docs/UnifiedHeaders.md).

There are other commits in buck to add support for r16b, so I was surprised to see these issues with headers in r16b. Is no one building with buck and r16b yet?