Disable always-on VPN options if an app has opted out.

The user can turn on / off always-on for individual VPN apps in
Settings > Network > VPN. Now that we're providing an opt-out of the
always-on feature for VPN apps, we should disabled the toggles if the
VPN app has an explicit opt-out.

Related unit tests are also removed as the check is now performed in the
frameworks, not in Settings.

Bug: 36650087
Test: CTS Verifier

Change-Id: I486fec14a1f8b86e7120dbdbfed3885801ab4dd7