CpuConsumer: Properly track acquired buffers

CpuConsumer cannot simply assume a slot's buffer is the same buffer
between acquire and release, and therefore it could be possible for
the same slot to get used for a second acquired buffer, if there's a
producer disconnect in between. This would cause a problem when the
first buffer is released by the consumer.

Instead, use an independent list of acquired buffers to properly track
their state.

Bug: 8291751
Change-Id: I0241ad8704e53d47318c7179b13daed8181b1fab