GrpclbLoadBalancer should have full control over the lifecycle of StopWatch

Would be good to add another `GrpclbLoadBalancer` constructor without the `StopWatch` argument. The old constructor is just `@VisibleForTesting`, and `GrpclbLoadBalancerProvider` will call the new constructor. In this way, `GrpclbLoadBalancer` can have full control over the lifecycle of the `StopWatch`.

Start by reading GrpclbLoadBalancer and GrpclbLoadBalancerProvider, focusing on the existing constructor and StopWatch ownership. Add the provider-facing constructor while retaining the testing constructor, then verify that the load balancer controls the StopWatch lifecycle as intended.

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