Simplify addRoutesToLocalNetwork calls

There are only three calls to this method, and two of them
are with a list of 1 address which means it's simpler inlined.

The last call will be replaced with calls to RouteController
in a followup, meaning we addRoutesToLocalNetwork can be removed.

Test: RouterAdvertisementDaemonTest
      NetdUtilsTest
Change-Id: I3b6ee1db40f09954b336321000735a66306bf000