E. F-Droid needs `submodules: true`

[sunkup]

After D the translations live in a git submodule, and F-Droid doesn't fetch submodules unless the build recipe says so. I checked, and ours currently doesn't.

https://gitlab.com/fdroid/fdroiddata/-/blob/master/metadata/at.bitfire.davdroid.yml?ref_type=heads

submodules: true was set on every build up to 4.3.4.1-ose and then dropped from 4.3.5-ose onwards - prob when the old cert4android/ical4android/vcard4android submodules were replaced by Gradle dependencies. The current entry (4.5.19) only has subdir, gradle and prebuild.

What the flag does, per the build metadata reference: "causes git submodule update --init --recursive to be executed after the source is cloned".

Note it's a merge request on GitLab, not a GitHub PR, and we don't control the review speed - that's why this should be done with prio.

• Merge request against fdroiddata adding submodules: true to the latest build entry in metadata/at.bitfire.davdroid.yml. Worth explaining in the MR that an upcoming release adds a translations submodule and we want the autoupdate template to carry the flag - at the current commit there are no submodules, so it's a harmless no-op until D lands.
• Make sure davx5-translations is public - F-Droid clones anonymously
• After the first release containing D, check the F-Droid build really picked the translations up (also covered in H)