Add StackedLinkProperties test case

Change access level of getNetworkAgentInfoForNetwork in ConnectivityService to get
better testability. And add test case to verify clat state transition and make sure
linkProperties are updated correctly.

Bug: 80261579, 109913107
Test: runtest frameworks-net -c com.android.server.ConnectivityServiceTest \
          -m testStackedLinkProperties
Change-Id: I72fce594d74bd349f65557edca03640e1b86317c