Expose UdpEncapsulationSocket.getResourceId()

ConnectivityManager.createSocketKeepalive() will need to get
the socket resource ID as a parameter for creating a new
NattSocketKeepalive. ConnectivityManager is a part of incoming
ConnectivityService mainline, so expose getResourceId() as an
API since the hidden APIs are not accessible for a mainline
module. This API should not be exposed to apps, so make it a
MODULE_LIBRARIES system API.

Bug: 172183305
Test: make update-api
Change-Id: Ic0722352ea186fcb18a2d91cc3969f771fde9e86