Add CancellationFlag::Uncancel and on calls to ClientProxy::AddCancellationFlag, if a flag is already in the map, uncancel it. This will address the case when users use NC to share/receive a file, then cancel in the middle because the wrong file was selected, and then re-do right after. Without this change, the second share/receive process will be seen as cancelled with cancellation flags enabled. However this change will uncancel the flag which is added in RequestConnection and OnConnectionInitiated in the NS flow.
PiperOrigin-RevId: 540633470