libgui: fix and clean up CpuConsumer::unlockBuffer

addReleaseFenceLocked should be called with
AcquiredBuffer::mGraphicBuffer, not the one in ConsumerBase::mSlots.
Otherwise, it breaks ConsumerBase::stillTracking.

Also thanks to stillTracking, we don't need to do the tracking
manually.

Test: libgui_test
Change-Id: Ib69b88263c4619634ae2977b9f811be5a846f446