allow system backups (low effort)

[mutagen12]

Steps to Reproduce

capyreader doesn't allow it's data to be backed up by system.
for example at lineage OS system backup component seedvault lists capyreader data is listed as forbidden to be backed up

Expected behavior

capyreader is listed as common application, allowing backups

perhaps, manifest flags and additional @xml/backup_rules
should be enough:
https://developer.android.com/identity/data/autobackup#EnablingAutoBackup

<application ...
android:fullBackupContent="@xml/backup_rules">
android:allowBackup="true"

@xml/backup_rules :

Device, Operating system, and app version

lineage os 18
lineage os 21