message: Increase ANDROID_NDK_API_LEVEL to 21 in Dockerfile
diff: @@ -7,7 +7,7 @@ RUN mkdir -p ${ANDROID_DEV_HOME}
  ENV ANDROID_SDK_FILENAME tools_r25.2.5-linux.zip
  ENV ANDROID_SDK_URL https://dl.google.com/android/repository/${ANDROID_SDK_FILENAME}
  ENV ANDROID_API_LEVEL 23
  -ENV ANDROID_NDK_API_LEVEL 18
  +ENV ANDROID_NDK_API_LEVEL 21
  # Build Tools Version liable to change.
  ENV ANDROID_BUILD_TOOLS_VERSION 28.0.0
  ENV ANDROID_SDK_HOME ${ANDROID_DEV_HOME}/sdk
  