cutils: Add support for BFQIO cgroups

 * Add support for a custom hierarchy of cgroups on top of the BFQ
   IO scheduler. This allows us to place every thread into the
   right class (realtime/best-effort/idle) with a set up priority
   buckets depending on use case.
 * The benefit of doing this is pretty incredible from an
   interactivity standpoint. Realtime users (display/audio) benefit
   the most, resulting in glitch-free audio and jank-free video.
   Dexopting in the background no longer causes active harm to
   foreground tasks. Other tasks such as account syncing become
   invisible from the user's perspective.
 * Magic bullet? Perhaps.

Change-Id: I4eb911395364ce46d6dcbff43e94286ded03a97d