Add a function to process LinkProperties coming from an agent.

There are tasks that need to be performed when receiving
LinkProperties directly from a NetworkAgent (either at
registration time or in subsequent updates).

Currently, the only example of such a task is calling
ensureDirectlyConnectedRoutes. This is currently done in
handleUpdateLinkProperties, which is often unnecessary,
because that method iscalled in many other cases than when
receiving properties directly from an agent. Ensuring directly
connected routes only needs to be done when receiving
LinkProperties from the agent, because ConnectivityService does
not directly manipulate routes.

This CL does not do much except remove these superfluous calls
and add the method. A future CL will add more code to the method.

Bug: 150648313
Test: atest ConnectivityServiceTest
Change-Id: Ibeeb5f79e8afd3350c935934713d7882f2e0281f