Update LinkProperties to NetworkAgentInfo only when it's changed

In previous design, it will always assign newLp to nai in
handleUpdateLinkProperties(). And Private dns configuration
will be missing when the same LinkProperties are updated
because the updated LinkProperties is not assigned back to
NetworkAgentInfo.

Bug: 118518971
Test: 1.Build pass.
      2.runtest frameworks-net

Change-Id: I405c8f29497fec438082a2cf30eb5c7b9497e1c4