Support gradle ABI splits

Gradle now supports ABI splits, e.g. look at https://f-droid.org/forums/topic/mitzuli-language-translator/ . A single build outputs multiple apks (with different versioncodes). Since our metadata is one build, one versioncode, we currently deal with it by messing with upstream's gradle files.

I'd like to propose that we have a field which explicitly states the expected output apk, maybe even a regex. Current behavior is to simply fail on multiple apks.

Can/should we re-use output= for this?