BufferQueueProducer: Signal consumer when a buffer is detached

When a buffer is detached, it should be cleared from the consumer-side
cache as well.

Not doing so can leak buffers no longer owned by the queue for extended
lengths of time, depending on the exact usage patterns of the producer
and consumer.

Bug: 28695173
Change-Id: I2c618be51f2c827d92d0e19b92a2d5befda87c38