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
Bug: 303408193
Test: manual, VpnTest
(cherry picked from https://android-review.googlesource.com/q/commit:cabbb7da639520633ad318655d1b5fe1c685c78e)
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:4a6567e229cdc01d12c91910b68579d42b06fc6d
Merged-In: I03399c7c258ce5db67e81b4e51f25f2cb37700c9
Change-Id: I03399c7c258ce5db67e81b4e51f25f2cb37700c9