You can import into the User CA store, not into the system CA store. Since 8.0 these are separate, apps have to explicitly opt in to using user CA certs as wel

You can import into the User CA store, not into the system CA store. Since 8.0 these are separate, apps have to explicitly opt in to using user CA certs as well. For example, Google's News app doesn't, and as result you can't ad block on the articles it shows.