[DEVOPS-849] Refactor baking PR number into APK

[csongorhg]

Currently .github/workflows/update_app_identifiers.sh is corrupting the created APK on CI. I propose to simply echo the PR number to a txt file and include that in the APK. This PR also unblocks creating corrupted APK on CI.

After downloading the apk, one can see the PR number such as:

unzip flutter-paint-131.apk
cat ./assets/flutter_assets/assets/pr_number.txt
131

Ticket: https://catrobat.atlassian.net/browse/DEVOPS-849?atlOrigin=eyJpIjoiNjJlNzQwYWQyZjU3NGQ0Njk3ZGIzYzUyZGNhOGM2OTYiLCJwIjoiaiJ9

Refactorings and Bug Fixes

• Unblock creating corrupt APK on CI

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