Disable Auto Backup on Android

Currently, on Android the app data is automatically backed up. However, this has several "issues":

• First of all, it does not play well nice with the app's claim that e.g. a local library's data never leaves the device (unless the user turned off backup at all).
• And second, not all data is backed up and even worse: The data set - if the user works a lot with e.g. images - quickly will grow beyond the 25MB limit (for more details, see https://developer.android.com/guide/topics/data/backup).

For this reason, we should disable auto backup for the app on Android and rather implement a backup mechanism which is available on all platforms.