sched_policy: Add support for /proc/<tid>/timerslack_ns over PR_SET_TIMERSLACK_PID

The PR_SET_TIMERSLACK_PID number has collided with upstream
changes a number of times, and thus the number is not consistent
between AOSP common kernel versions.

In 4.6+, the upstream kernel added equivalent functionaity via
the /proc/<tid>/timerslack_ns entry.

This patch changes the sched_policy logic to support
/proc/<tid>/timerslack_ns if its available, falling back to the
older PR_SET_TIMERSLACK_PID method if not.

NOTE: Eventually PR_SET_TIMERSLACK_PID usage will need to be
removed as it is likely to conflict with valid future upstream
PR_* entries.

Also add missing O_CLOEXEC to other open calls in this file.

Change-Id: Iec0b8a62de0dc8bdd57b60df82bd4d31c5d64709
Signed-off-by: John Stultz <john.stultz@linaro.org>