The native VM service disconnects all non-DDS clients when DDS connects
in order to keep state consistent. DWDS was never configured to work
this way and, instead, would not let DDS connect if there were already
existing clients.

This is fine most of the time, as DDS is typically the first client, but
in cases where multiple DDS instances attempt to start at once (e.g., a
simultaneous `flutter run` and `flutter attach`), this inconsistency
prevents tools from falling back to using an already connected DDS
instance, as seen in flutter/flutter#171758.

Fixes #2399

…ted clients (#2679)

This is a regression from 1c29125 and was reported in flutter/flutter#174437