update: more accurate fastlane subdir/flavor matching

This should fix Schildichat showing Element X's metadata. The problem is that my original test was too lenient, and it falsely let upstream_infra/fastlane/metadata/android/en-US/ through as a subdir: value.

And yeah, the fastlane code is whacky and should be rewritten. The problem is that operates on last match is winner take all. It should be first match is winner take all. The Fastlane tool itself has rules for finding the fastlane/ dir, I think we should implement their algorithm.