surfaceflinger: writepid to the actual stune mount point

Seems like AOSP missed these out when switching the schedTune
cgroup mount point from /sys/fs/cgroup/stune/ to /dev/stune/.

Reference commit:
11cde56 Mount schedTune cgroup as /dev/stune

Change-Id: Ifbd9ddcaab0fa2a55433a9a66e9a907b6c242204