sched: compute load contribution by a group entity

Unlike task entities who have a fixed weight, group entities instead own a
fraction of their parenting task_group's shares as their contributed weight.

Compute this fraction so that we can correctly account hierarchies and shared
entity nodes.

Signed-off-by: Paul Turner <pjt@google.com>
Reviewed-by: Ben Segall <bsegall@google.com>