lisa.analysis.tasks: Add df/plot_task_activations()
lisa.analysis.tasks: Add df/plot_task_activations()

Give a kernelshark-like plot of task activations, i.e. high when it is
runnable, low when sleeping.

plot_task_activations() can deal with an axis with existing data: It
will adjust the height to take 1/4 of the range, and will use 50%
transparency to allow seeing the other curves.

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