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