gitlab-ci: Switch to chrome_public_apk as target

- chrome_modern_public_apk was deprecated in [1] leaving chrome_public_apk
  as the alternative choice as recommended in docs

References:
- [1] https://chromium.googlesource.com/chromium/src.git/+/cdbc63e812e584b916444b13e67543e3e8924f8e
- [2] https://chromium.googlesource.com/chromium/src.git/+/HEAD/docs/android_build_instructions.md#multiple-chrome-targets



Test: Build chrome_modern_public_apk and notice that it fails with unknown target error

Signed-off-by: Aayush Gupta <theimpulson@e.email>