To decouple VpnManagerService from ConnectivityServiceTest, stop
calling setAlwaysOnPackage from VpnManagerService as a
preparation for the follow up changes. Update it to equivalent
calls from Vpn directly.
The VpnManagerService variable is still needed because tests
have indirect calls via sending intent to update the lock
down behavior.
Bug: 230548427
Test: atest ConnectivityServiceTest
Change-Id: Ib78ac8173b2a4d2a8e9845e943eb63d79c36218c
3797f878ac