Add ConnectivityManager.TYPE_PROXY to SystemApi

This follows other TYPE_* constants like TYPE_WIFI_P2P that are
@SystemApi or public.
TYPE_PROXY has a use-case for the system to set network policies based
on proxy network templates. Although network types are deprecated, that
use-case needs to be supported and significant amounts of network
management would need to be rewritten to stop using network types.

The constant needs to be API as ConnectivityManager is planned to move
out of framework.jar, so only its formal API will be available to the
system server.

Bug: 174436414
Test: m
Change-Id: I266ed6bc59f5eb72302afe14472c93933733c8f8