[ DDS / Service ] Put the VM service into single client mode when DDS connects

Added a private RPC, _yieldControlToDDS, which DDS invokes once
connected to the VM service. Once this method is called, the following
happens:

- The VM service stops accepting new web socket requests.
- The VM service enters single client mode, where DDS is the single
  client.

DDS can only place the VM service in single client mode when it is the only
VM service client. If other clients are connected, DDS will fail to start and
throw an exception.

The purpose of this change is to simplify state management between the
VM service and DDS. The plan is to delegate the following features to
DDS:

- Service extension management
- Custom client name support
- Isolate resume coordination for multiple clients

Change-Id: I0b3143710d94ee9444a4894e24555f7ed1caf5ad
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/142564
Commit-Queue: Ben Konyi <bkonyi@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>