Hi,
The last ndk update is from before the release of Android 5.1, that's why there is no platform-22 inside it.
But if you compile for android-21 or any earlier platform, it will work on Android 5.1 devices.
There has been new NDK APIs/features introduced with android-21, such as new media APIs, support for OpenGL ES 3.1... but I haven't heard about any NDK specific changes coming with android-22, why would you need to compile against the android-22 platform ?
Regards,
Xavier.