Add tethering native flag namespace to sDeviceConfigScopes

DeviceConfig was moved to the module and adding new namespace requires
API change but API is already frozen.
So this CL defines namespace constant in SettingsToPropertiesMapper.
This constant needs to be moved to DeviceConfig module (b/282593625)

Bug: 281944942
Test: adb shell device_config put tethering_u_or_later_native test 1
Test: Read persist.device_config.tethering_u_or_later_native.test property
Test: from system server and Tethering.apk
Change-Id: I6d275f3943c14c65b05d81e3049b4757d9063806