sched/fair: Use util_burst for util_est updates

Util_burst allows to have an estimation of the size of a task
independently from co-scheduled tasks and their nice value. It
provides a utilization value as if a task had not been sharing the
rq with other tasks.

This means that util_burst:
- reaches a higher value than util_avg at dequeue
- fits the purpose of util_est better than util_avg

Make use of the newly added util_burst signal to update util_est.