Don't enforce control permission when preparing consented VPN.

If a VPN app requests to be prepared and has already obtained user
consent, there is no need to additionally enforce the control
permission. We only need to enforce the control permission when a VPN
is first being prepared, where such a preparation would bypass user
consent.

Also ensure that in this case, the VPN being prepared matches the
calling app. Otherwise an app could prepare another pre-consented VPN,
which is not particularly dangerous but is likely unexpected.

Finally, remove misleading comment in ConnectivityService#prepareVpn.
This method IS called from VpnService.prepare(), not only from
system-privileged apps.

Bug: 18442887
Change-Id: Ic3227c6c1c74312697f0576d3811b06692a4edff