Change get_sched_policy to check "schedtune" and "cpuset".

get_sched_policy will first attempt to get the policy from the "schedtune"
subsystem cgroup and, if not set, attempt to get the policy from the
"cpuset" subsystem cgroup.  If neither subsystem has a cgroup set,
SP_FOREGROUND is returned.

Bug: 32972117
Test: Verified that SchedPolicy libcutils test passes and that ps displays
the expected policies.  See details in testing done comment in
https://android-review.googlesource.com/379426 .

Change-Id: I586a921a38eea99d65590b07ece96c9808a2e56d