SocketUtils contains system APIs for modules to interact for sockets,
wrapping internal APIs. It should be part of the platform to keep access
to the internal APIs.
This involves splitting NetworkUtils.protectVpn to NetworkUtilsInternal,
since SocketUtils and VpnService are the only users of that method.
The @UnsupportedAppUsage NetworkUtils.protectVpn has low usage
count, and is already available through VpnService.protect.
Bug: 181512874
Test: boots, VPN working
Change-Id: I7028d334975f7536c06afac7a22200c33db707ac
fc6869064c