After building UE4.21.1 from source on a host Ubuntu 18.0.4 LTS system, and following the instructions on the unreal website to build an example package on Linux for Android, I received an error:
LogPlayLevel: Error: ERROR: Android toolchain NDK r12c not supported; please use NDK r14b to NDK r18b (NDK r14b recommended)
which other users got in a different situation, here.
This is because the version of Nvidia Codeworks packaged with the current release branch gives the Android NDK version 12b.
To fix this, I installed Nvidia codeworks 1R7 which includes Android NDK version 14, and I was able to fully build and deploy an example application on my Android device (after fixing my JAVA_HOME environment variable which might have gotten mangled by the install scripts). I was not able to successfully install Nvidia codeworks 1R8.
I hope this helps anyone else who runs into the same issues.