Liseur: publish developer-signed APKs alongside F-Droid-signed ones (reproducible builds)

Liseur builds reproducibly from v0.9.3 (versionCode 17). This MR sets up dual signing, as suggested in the review below: F-Droid keeps signing for existing users, and new users get the APK carrying my signature.

• AllowedAPKSigningKeys: pins my certificate. I checked the digest against the published APK with apksigner verify --print-certs.
• metadata/com.chmouel.liseur/signatures/17/ contains the signature extracted from the v0.9.3 release APK with fdroid signatures (v2+v3 only, made with apksigner).
• A build entry for 17, and AutoUpdateMode: None. I'll open an MR with the extracted signatures for each release.

Reproducibility landed after the v0.9.2 tag (chmouel/liseur#54: the APK used to carry the Play dependency-info blob, and its native libs were re-stripped by whatever NDK the build machine had). 17 is the first version that verifies. 13 and 16 are still queued from before and will publish F-Droid-signed as usual. From 17 on, the release script builds every tag twice from clean checkouts and refuses to tag unless the two APKs come out byte-identical.