Do null-checks before accessing VpnManager in DevicePolicyManagerService

Wear devices do not start the VpnManagerService, which would make
VpnManager unavailable. Thus, we now need to check if the VpnManager is
available before trying to use it.

Bug: 352669332
Bug: 340928692
Test: manual
Flag: android.server.allow_removing_vpn_service
Change-Id: I0e6ce00a6e5a87f983720656e5f11369d76ecfc1