Consider exclusion of data encryption keys from auto backup

[mottljan]

Tink generates and stores data encryption keys to a shared preferences file. Clients can change the alias and even the file where the keys are stored, but we might exclude at least the default file from autobackup in the library, so clients do not have to care about this. It is documented that clients should exclude these files from autobackup (since Jetpack Security lib), but would be better to do it automatically where possible.