App uses wrong NDK

It seems cmake always used the latest version.

build log of se.leap.riseupvpn:155

> Task :main:generateJsonModelSkeletonRelease
/home/vagrant/build/se.leap.riseupvpn/ics-openvpn/main/src/main/cpp/CMakeLists.txt : C/C++ skeletonRelease|armeabi-v7a : CMake Warning at /opt/android-sdk/ndk/22.1.7171670/build/cmake/android.toolchain.cmake:449 (message):
  An old version of CMake is being used that cannot automatically detect
  compiler attributes.  Compiler identification is being bypassed.  Some
  values may be wrong or missing.  Update to CMake 3.19 or newer to use
  CMake's built-in compiler identification.
Call Stack (most recent call first):
  /opt/android-sdk/cmake/3.10.2.4988404/share/cmake-3.10/Modules/CMakeDetermineSystem.cmake:94 (include)
  CMakeLists.txt

NDK r22b is used though r20b is set. Maybe we should remove unused NDK is no NDK is set. It also causes errors when there is binary need to be striped.