sched_policy: add get_cpuset/sched_policy_profile_name

Expose API to return name of task profile for cpuset/sched policy so
that libprocessgroup clients using SetTaskProfiles directly don't have
to maintain the mapping. This reduces the risk of inconsistency and saves
memory.

Bug: 139521784
Test: atest libcutils_test:SchedPolicy
Change-Id: I414312a038613913fb6a827bdcefceb3dec21264