In drmemtrace scheduler, track prior state to mark idle-to-input transition as thread vs process switch

Today the drmemtrace scheduler treats idle-to-input as a process switch:
```
if ( // XXX: idle-to-input transitions are assumed to be process switches
// for now. But we may want to improve this heuristic.
```

But this is likely wrong most of the time since we usually run a single workload.
We should track the prior input to know whether this is a thread or process switch.