[dds] Ensure thread startup is not handled twice in DAP when attaching
[dds] Ensure thread startup is not handled twice in DAP when attaching

Should fix the flakes seen in https://github.com/dart-lang/sdk/issues/48274 caused by Isolate PauseStart events arriving during initialization but being delayed until initialization complete, and then auto-resuming isolates that ere deliberately being left paused.

Change-Id: Iee32731b92fe96b6b41e7e78c01af2f5e843ea92
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/231460
Reviewed-by: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>
Commit-Queue: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk