Move SchedulingPolicyService init code to SystemServerInitThreadPool

To avoid slowing down service start time.

Also restrict cpuset change api use to mediaserve only.

bug: 78307194
Test:
- boottime test in go/atest-perf:
  atest google/perf/boottime/boottime-test

- Manual testing: play 1080pHDR content in modified exoplayer (that
  uses soft MediaCodec), verify media.codec is put into top-app; then
  kill system_server process during playback (using adb shell stop
  && adb shell start), and verify after restart that media.codec
  is put back into fg.

Change-Id: If761bd75a7ed893811db3f44bfd84ee3095e23e2