Support Android data backup API

[phw]

This would backup the list of website URLs.

See http://developer.android.com/guide/topics/data/backup.html

This would be useful for manual backup via ADB as Helium Backup [1] does. I think for this to work it is not necessary to register the App for the Android Backup Service [2](which would cause a cloud backup of eventually sensitive data).

[1] https://play.google.com/store/apps/details?id=com.koushikdutta.backup&hl=de
[2] http://developer.android.com/guide/topics/data/backup.html#BackupKey