libgui: Plumb attach/detach through CpuConsumer

Adds CpuConsumer::{detachNextBuffer,attachAndReleaseBuffer}, which
can be used to more carefully manage the ownership of GraphicBuffers.

Bug: 19628705
Change-Id: Ia7aa1ac59c2f768f2d8a0f35ad23067936a7427c
3 files changed