Use async approach to change sched policy

Currently, sched policy of a thread may be changed when setting
its priority. However, the sched policy change may fail because
of the limitation of sepolicy. To solve it, use async approach
to let activity manager service change the sched policy of the
thread.

Bug: 139521784
Test: function works as expected
Change-Id: I967723e8f7ad95c76fd7efdc952d6f7b579b1d61