addonsu: Fix installation on Lineage recovery

Commit fd798c51 restored the old TWRP
workaround, which installs addonsu stuff to /system/system for
compatibility reasons.

However, our script currently (and correctly) mounts /system_root/system
to /system, meaning the actual "system" contents are properly presented
at /system. This breaks addonsu installation because of the TWRP workaround.

Now we could just revert the TWRP workaround again and break all users on
such setups, but instead we decided to add yet another workaround to make
the package compatible with both TWRP and Lineage recovery systems.

Change-Id: I7f44a29c49e07e59017d8eba7359e3f8d8b5a4f7