Change set_sched_policy to set slack for current thread.

Change set_sched_policy to use prctl PR_SET_TIMERSLACK if setting the policy
for the current thread and /proc/<tid>/timerslack_ns is not supported by the
kernel.

Bug: 32972117
Test: Verified that libcutils SchedPolicy tests pass.  See details in
testing done comment in
https://android-review.googlesource.com/381303 .
Change-Id: Id70940ed7f9ed94c3d105213f069bf3e5a8d8824