Modify LinkProperties add/remove route functions.

This will allow us to dynamically track routes being added and
removed, similar to what we do for IP addresses.

1. Support removing routes. Since this is a new function, we
   don't need to jump through hoops to support callers passing
   in routes that have no interface, we just fail to match them.
2. Make the addRoute method return a boolean value indicating
   whether anything changed. This is consistent with what we do
   for addresses and is used to decide whether to update the
   rest of the system when an update comes in.

Bug: 9180552
Change-Id: I50648b5f81ec55c88501a7640e119cda2bb540f2