Support losing the upstream in IpServerTest.

Allow dispatchTetherConnectionChanged to pass in a null
upstream interface and/or null LinkProperties. This will allow
future tests to check what happens when the upstream interface
goes away.

Bug: 149963652
Test: test-only change
Change-Id: I7cfc32d072f23644159d33700d6cd830c426ea19
Merged-In: I7cfc32d072f23644159d33700d6cd830c426ea19
(cherry picked from commit 62457f17)