sched: ems: service: fix service CPU selection algorithm

1. Consider boosted task utilization when choosing/considering
maximum spare capacity
2. Take advantage of CPU cstates
3. Account of task utilization to avoid overutilizing a CPU

Signed-off-by: Diep Quynh <remilia.1505@gmail.com>
Change-Id: I4c5731c7ce5caaac59fb9a8f98c139359f7e54f3