lisa.tests.scheduler.load_tracking: Fix CPU active time accounting
lisa.tests.scheduler.load_tracking: Fix CPU active time accounting

Make sure we get the average duty cycle pro-rated to the time spent on each CPU
for a given phase. Otherwise, a single activation on the wrong CPU before
migration will be assumed to be the average duty cycle on that CPU for the whole
phase.

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