Add metrics logging for B&R of locale data in settings.

This is part of an effort to add metrics support to SettingsBackupAgent. For more details, see go/settings-backup-agent-metrics-design

Bug: 379861078
Change-Id: I234549f11a1a2e6296e8611aa2bc973019be3b92
Flag: com.android.server.backup.enable_metrics_settings_backup_agents
Tested: SettingsHelperTest and manually by verifying that the metrics are logged during backup (https://screenshot.googleplex.com/9gkq9bUtXXyaVXp) and restore (https://screenshot.googleplex.com/BqnQFn2CBguLfU6). Note that I tried adding tests for more methods in SettingsHelperTest, but I couldn't due to an issue with permissions when calling `am.updatePersistentConfigurationWithAttribution`.