Stop BdxOTASender polling timer after EOF on Darwin

[pgravel-com]

Polling timer is never stopped after successful file transfer when using src/darwin/Framework/CHIP/MTROTAProviderDelegateBridge.mm

Step to reproduce -
Add log to BdxOTASender:HandleTransferSessionOutput for visibility
Initiate BDX OTA firmware update transfer
Wait for EOF
See timer still firing after EOF and log getting printed.

15:39:53.310 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kAckEOFReceived
15:39:53.317 E  [BDX] OnTransferSessionEnd
15:39:53.317 E  [BDX] BdxOTASender::Reset
15:39:53.378 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:53.437 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:53.496 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:53.553 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:53.605 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:53.661 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:53.717 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:53.770 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:53.825 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:53.887 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:53.952 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:54.013 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:54.080 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:54.144 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
15:39:54.204 E  [BDX] BdxOTASender::HandleTransferSessionOutput - kNone
...

As per https://github.com/project-chip/connectedhomeip/blob/master/examples/ota-provider-app/ota-provider-common/BdxOtaSender.cpp#L183 , mStopPolling should be set to TRUE on AckEOF or possibly in Reset().

[bzbarsky-apple]

@vivien-apple