Before NDK r14, we had a set of libc headers for each API version. In many cases these headers were incorrect. Many exposed APIs that didn‘t exist, and others didn’t expose APIs that did.
In NDK r14 (as an opt in feature) we unified these into a single set of headers, called unified headers. This single header path is used for every platform level. API level guards are handled with #ifdef. These headers can be found in [prebuilts/ndk/platform/sysroot].
Unified headers are built directly from the Android platform, so they are up to date and correct (or at the very least, any bugs in the NDK headers will also be a bug in the platform headers, which means we're much more likely to find them).
In r15 unified headers are used by default. In r16, the old headers have been removed.
See the Build System Maintainers doc.