sched: separate capacity margin for boosted tasks

With the introduction of placement hint patch, boosted tasks will not
scheduled from big cores. We tune capacity margin to let important
boosted tasks get scheduled on big cores. However, the capacity margin
affects all group of tasks, so that non-boosted tasks get more chances
to be scheduled on big cores, too. This could be solved by separating
capacity margin for boosted tasks.

Bug: 147785606
Test: margin set correctly
Signed-off-by: Rick Yiu <rickyiu@google.com>
Change-Id: I2b02e138e36a6844afbc1ade60fe86a001814b30