ci(android): let pull requests from forks build

[cuong-tran]

What & why

Every pull request from a fork fails "Build release APK (FOSS)", in both build and build-vivo, while pushes to dev pass. GitHub gives no repository secrets to pull_request runs from forks, so local.properties gets an empty SENTRY_AUTH_TOKEN. The Full build still had autoUploadProguardMapping on, so after the APK had already built, uploadSentryProguardMappingsRelease failed:

sentry reported an error: Invalid token header. No credentials provided. (http status: 401)

Seen on #2709 (run 39217097153) and on earlier fork PRs (runs 38939224887, 35792460797).

• androidApp and vivoAndroidApp: upload the mapping only when a token is present. Without one, the Sentry plugin runs sentry-cli upload-proguard --no-upload, which still writes the mapping UUID but makes no API call. Builds with the secret behave exactly as before. A local Full build without a token now succeeds too.
• android.yml: skip signing and the artifact upload for pull requests from forks, which have no signing key either. Pushes and same-repo PRs still sign and upload.

Trade-off: if the SENTRY_AUTH_TOKEN secret ever goes missing upstream, the mapping upload is skipped silently instead of failing the build.

Linked issue

No issue: this fixes the CI failure seen on fork PRs such as #2709.

Checklist

• This PR addresses an accepted issue (linked above)
• I wrote or personally reviewed every line of this change
• I tested it on Android and/or Desktop (say which, and how)

CI only, no app code changed. This PR comes from a fork, so its own check run is the test: both jobs should pass, with the sign and upload steps skipped.