Ideally if you took a Release .apk, anything in the lib/ABI should go into the respective /system/lib folder.
For example with a File->New Android Project
If I created a Release Aligned .apk, I should be able to extract it and view the following:
lib\armeabi-v7a
Which has two .so files here:
libmonodroid.so
libmonosgen-2.0.so
You will need to ensure that both of these .so
files are included in the /system/lib
folder and then your application should install as a system application.