load_tracking: rewrite get_task_duty_cycle_pct

Rewrite get_task_duty_cycle_pct using task analysis methods. It seems
that the Bart method for obtaining a tasks's run time on a CPU
sometimes fails to provide a correct value.

Given that we're using task analysis methods on all other tests with this
function being the only one remaining using a Bart (SchedAssert) method,
rewrite this function to use a more reliable method to obtain a task's
run time.

Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>