Revert "Remove cutils threads.h header completely."

This reverts commit d3550e33.

Reason for revert: DroidMonitor-triggered revert due to breakage https://android-build.googleplex.com/builds/quarterdeck?branch=git_tm-qpr-dev-plus-aosp&target=aosp_sunfish-userdebug&lkgb=10667572&lkbb=10667604&fkbb=10667601, bug http://b/296247274

Change-Id: I3f3864dc21fcb34cbb2ff524c43c3d92bd260ff6
BUG: 296247274