lisa.analysis.tasks: Fix df_task_activation() duty_cycle column
lisa.analysis.tasks: Fix df_task_activation() duty_cycle column

Correctly handle duty cycle computation by iteratively ignoring the preempted
rows, and merging their duration into the previous row. This means the duty
cycle will always be correct, no matter how many preemption happen during a
given activation.

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