Stop accessing VPNs in checkConnectivityDiagnosticsPermissions.

Currently, checkConnectivityDiagnosticsPermissions takes the VPN
lock to examine the VPN's underlying networks. Use the underlying
network data that is available in ConnectivityService instead.

Bug: 173331190
Test: passes existing tests in ConnectivityServiceTest
Change-Id: Ia1c366c5e9974d4d2c4b38030e66c007d62020ff