Add a new IpPrefix class and use it in RouteInfo.

This change uses IpPrefix only in the public API and continues
to use LinkAddress for everything else. It does not change the
callers to use the new APIs, with the exception of changing
all current uses of getDestination to getDestinationLinkAddress
to make room for the new getDestination method that returns an
IpPrefix.

Based on Sreeram's earlier change:
https://googleplex-android-review.git.corp.google.com/#/c/477874/
but a bit simplified and with a bit more documentation.

Bug: 15142362
Bug: 13885501
Change-Id: Ib4cd96b22cbff4ea31bb26a7853989f50da8de4e