lisa.analysis.tasks: Make df_tasks_runtime() computation more accurate
lisa.analysis.tasks: Make df_tasks_runtime() computation more accurate

Make sure to only account the time spent active inside the trace's window. We
want the window's left edge's row to propagate its status, but not its delta
since it's outside of the window.

credp/lisa,credp/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,ARM-software/lisa