Add Backup & Restore Metrics for Wifi settings.

Note that I couldn't add a test getWifiSettingsBackupData() because I was unable to mock WifiManager.retrieveWifiBackupData().

Bug: 379861078
Change-Id: Ie717f5b543bddd07fab8ba7ab24e5db64acad8c5
Flag: com.android.server.backup.enable_metrics_settings_backup_agents
Tested: atest SettingsBackupAgentTest