libcutils: Move sched_policy functions into libprocessgroup

Move functions operating on cgroups from sched_policy.h in libcutils
into sched_policy_ctrl.h under libprocessgroup. This consolidates
cgroup-related functionality inside libprocessgroup.

Bug: 111307099
Test: builds, boots

Merged-In: Iba75f33281162b889989214d0325a5973d53ed2d
Change-Id: Iba75f33281162b889989214d0325a5973d53ed2d
Signed-off-by: Suren Baghdasaryan <surenb@google.com>