Remove the UpdateAddresses() method from balancer.SubConn interface

[easwars]

This is the second and last change as outlined in this issue

Summary of changes:

• Remove the UpdateAddresses() method from the balancer.SubConn interface
• Unexport the UpdateAddresses() method on the acBalancerWrapper type.
• Store the concrete type *acBalancerWrapper in addrConn instead of the balancer.SubConn interface
• Remove no-op implementations of this method from fake/test subConn implementations used in tests

Fixes #4207

RELEASE NOTES:

• balancer: remove UpdateAddresses() from SubConn interface; use ClientConn.UpdateAddresses instead. See Upcoming experimental Balancer API changes #4207 for details.

[dfawley]

Let's discuss this further offline first.