core: remove redundant SubchannelPicker refreshes in RoundRobinLoadBalancer

* Remove redundant SubchannelPicker refreshes in RoundRobinLoadBalancer

- Ensure active subchannel list and round-robin index is only
regenerated/refreshed when it changes
- Make it so that Subchannels exist in subchannels map iff their state
!= SHUTDOWN
- Add EmptyPicker class since logic for this case is disjoint from the
non-empty case

* remove explicit initialization of boolean ready field

per @carl-mastrangelo's review comment

* minor restructuring to make logic clearer; more explanatory comments

* move some checks inside updateBalancingState method for clarity

* store current state and picker in RRLB, only update when new one is diff

* some more simplification/refactoring; improve test coverage

- remove now redundant check in handleSubchannelState

- collapse getAggregatedState() and getAggregatedError() into
handleBalancingState()

- have both pickers extend new RoundRobinPicker, move
areEquivalentPickers() logic into RoundRobinPicker.isEquivalentTo()

- extend unit tests to cover some additional cases

* Address latest review comments from @zhangkun83

- Use explicit check for non-empty list instead of assert
- Change EmptyPicker.status to be non-nullable
- Further test coverage improvement including explicit picker comparison
tests

* use EMPTY_OK instead of Status.OK for initial empty picker