xds: use locality from the connected address for load reporting

[townba]

• Add SubConnState.connectedAddress field.
• Add internal.GetConnectedAddress and internal.SetConnectedAddress accessor function vars to get and set SubConnState.connectedAddress.
• Set the connected address in acBalancerWrapper.updateState when connectivity state is Ready.
• Read the connected address in the subconn StateListener and call updateLocalityID() based on that address's locality.
• Modify addrConn to store the full resolver.Address rather than one with BalancerAttributes removed.
• Modify a test to check for the new correct behavior.

Fixes #7339

RELEASE NOTES:

• TBD