MINOR: activity: make the profiling status per thread and not global

MINOR: activity: make the profiling status per thread and not global

In order to later support automatic profiling turn on/off, we need to
have it per-thread. We're keeping the global option to know whether to
turn it or on off, but the profiling status is now set per thread. We're
updating the status in activity_count_runtime() which is called before
entering poll(). The reason is that we'll extend this with run time
measurement when deciding to automatically turn it on or off.