Fix addRoute replace default route unexpectedly

In aosp/1203789, if two routes are with the same destination,
it will be replaced instead of added when calling addRoute.
This breaks scenarios which rely on the ability to add multiple
default routes, such as multiple IPv6 default routes learned
via address autoconfiguration.

This change treats the route is an update if the destination
and nexthop are the same, but different in other properties.

Test: atest OffloadControllerTest#testSetUpstreamLinkPropertiesWorking
Test: atest LinkPropertiesUtilsTest#testLinkPropertiesIdenticalEqual
Test: atest ConnectivityServiceTest#testStackedLinkProperties
Test: atest ConnectivityServiceTest#testRouteAddDeleteUpdate
  (only directly related tests are listed)
Fix: 152170074
Fix: 151911339
Bug: 142892223

Change-Id: I7153ec9866f14a109ba8155c905e5d9e4f85eb64