lisa.analysis.tasks: Fix plot_task_activation(): do not extend activations
lisa.analysis.tasks: Fix plot_task_activation(): do not extend activations

Do not extend the validity of the last event in the selected window beyond the
end of the dataframe. This avoids extending an activation, preventing a "forever
activated" task resulting in a solid rectangle being drawn.

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