Move keys out of SettingsBackupAgent so that they can be reused.

With all the logging that I'm adding in SettingsHelper, I think it'd be useful to have these constants defined out of the agent so that they can be reused.

Bug: 379861078
Change-Id: Icbd0b926eba981c0492bf4e72e9e5cf6f40104b9
Flag: com.android.server.backup.enable_metrics_settings_backup_agents
Tested: SettingsBackupRestoreKeysTest