Signing android CI build

[pkongkha]

When downloading artifact from CI builds, adb install app-release.apk says

adb: failed to install app-release.apk: Failure [INSTALL_PARSE_FAILED_NO_CERTIFICATES: Failed to collect certificates from /data/app/xxx.tmp/base.apk: Attempt to get length of null array]

Here's the picture from @Anton-Dodonov when trying to install from android devices

The problem occured when trying to install unsigned apk
For reference: https://stackoverflow.com/questions/2914105/what-is-install-parse-failed-no-certificates-error

Because there is no key.properties in android/ directory, so the apk is unsigned
https://github.com/VPNclient/VPNclient-app/actions/runs/14893078302/job/41829656121#step:5:70