Add __ANDROID_NDK__ to identify using the NDK.

I've added some things like __ANDROID_MAJOR__ to an ndk-version.h, but
that is only in the NDK itself and so doesn't help the platform. Add
__ANDROID_NDK__ to identify that you're building for the NDK and not
the platform.

Test: make checkbuild
Bug: https://github.com/android-ndk/ndk/issues/407
Change-Id: I2d1f1c28e3764e4e658cf675b290b7a17253ee33