'mk_add_options' or 'export' MOZILLA_OFFICIAL

In #27472 (moved), we added export MOZILLA_OFFICIAL=1 in .mozconfig-android because we found the compiled APK wasn't releasable (it was still debuggable). This was due to MOZILLA_OFFICIAL not being set correctly at build-time. After exporting MOZILLA_OFFICIAL, the resulting APK is for release. Do we need both mk_add_options and export or do we only need export? Should we use export on the desktop builds, too?

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