support manually adding per-Build Anti-Features in metadata, and other fixes
This makes it possible to specify the Anti-Features as part of the Build, instead of for the whole app. This more accurately reflects the situation, for example, an app can add or remove tracking, so each Build should be marked accordingly. This also makes it possible to mark Builds with KnownVuln, for example in the case of the recent OpenVPN vulnerabilities.
There are a couple other small fixes, for example, it should make it easier to update the NDK version, like in !293 (merged)