Add check to prevent resetting VPN always-on setting in safe mode

This change prevents the VPN always-on setting from being reset
in safe mode, where third-party VPNs may have limited
functionality in safe mode and can be deemed unsupported for
always-on use.

Bug: 397215751
Test: manual, VpnTest
Change-Id: I03399c7c258ce5db67e81b4e51f25f2cb37700c9