Revert "Move sched_policy.c off bionic's PR_SET_TIMERSLACK_PID."

This reverts commit 8f81759dde0baef4e7a00bdfeba7625476626784.
Revert "Use <sys/prctl.h> instead of <linux/prctl.h>."

This reverts commit 14c603c5b73173f01191b820092638527b6f2624.