First mentioned in another comment, and then discussed with @ejona86 and @dapengzhang0, it was concluded that we need to extract the following logic:

1. Parse the target string and convert it into a URI
2. Manage NameResolver retries
3. Wire up NameResolver and LoadBalancer

out of ManagedChannelImpl, and make it usable in the following cases:

1. CHANNEL: ManagedChannelImpl
2. OOB: LoadBalancer.Helper#createResolvingOobChannel (blocking Implement LoadBalancer.Helper#createResolvingOobChannel #5495)
3. CHILD_POLICY: XdsLoadBalancer "custom mode": XdsLoadBalancer receives the target name for each locality, and needs to resolve the target name and start a child LoadBlanacer for it.