Store TPC cluster flag in UnbinnedResid::channel

[shahor02]

The TPC cluster flags are now stored in the residulas channel word. Before it was set to -1 for TPC residuals and provided the detector channel info for ITS, TRD, TOF residuals (row>=160).
Now it is assigned for TPC residuals as:

short channel = cluster.getFlags();
if (TPCCusterSharingMap[clusterIndex] & o2::gpu::GPUTPCGMMergedTrackHit::flagShared) {
  channel |= o2::gpu::GPUTPCGMMergedTrackHit::flagShared;
}

Tested on OO data: