Regarding about build systems:
from now Android 7.1 (Nougat) build system is obsolete, this means it won't get any new updates. If your device using the nougat build system you should update it to Oreo/Pie as fast as possible.
Soon we will make final Nougat's source repo/manifest.

Also, we have updated the Android 8.1 (Oreo) build system and fixed syncing sources as well.