Hide mutable VpnService APIs to add/remove IP addresses dynamically.

These APIs were added because we thought we needed them to provide
seamless transition from one server backend to another using local IP
addresses to distinguish between the backends. I.e., connections whose
local IP address was old would be routed to the old backend; connections
whose local IP address was new would be routed to the new backend.

It turns out that's not needed. VpnService already supports seamless
re-establishment, so VPNs just need to call establish() again with a
different IP address. I've verified with a custom VPN app that this
works, and can distinguish traffic based on the old and new addresses.

Nobody is using these APIs at the moment, so we could even consider
removing them altogether, but I prefer just hiding them, just in case.

Bug: 15409819
Change-Id: I30949926a0f859c9d839981ccbc5d8e1e535a3a5