Investigate CMake build reliability

[ephemer]

Often when we build for Android it's unclear whether the APK contains the latest .so libraries. Also, after cleaning the APK, the first build always fails because the native libraries are missing.

It's possible these are all just Android Studio bugs, but it does seem likely that this could be enforced with a more "integrated" CMakeLists.txt process: at the moment we are manually coping the .so library files into the output directory. Maybe there's a better way to go about this.