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- Now that it's possible to resize a BufferQueue while buffers are
  dequeued/acquired, it's no longer correct for Surface to clear its
  cache when the BufferQueue is resized since it must keep at least
  the currently dequeued buffers.
- Add an onSlotsFreed callback to IProducerListener so that producers
  that wish to be notified about buffers being freed can do so. Note
  that this isn't currently used in Surface.
- Review and fixup all the places where the producer/consumer
  listeners for freed buffers should be called.

Change-Id: I4ab0c92bc69b75a3c072ddf5d74d78f941dba4c8