Allow backup in AndroidManifest.xml

[varistov]

AndroidManifest.xml says Backup of apps is not
allowed android:allowBackup="false"

android:allowBackup should be set to "true" to allow backup of foxydroid app data by seedvault

This is very much useful when you use custom repos and you need backup. Fdroid classic does allow backup.

[kitsunyan]

I've never used backup on Android, and there are much more options than just allowBackup in manifest. I think I'll need to study and test them before enabling backups.