Return pdx::Status<T> from BufferHubQueue::Dequeue.

Switch to using Status<T> to return buffers or meaningful errors
from BufferHubQueue::Dequeue. This enables determining whether an
error is normal (e.g. timeout) or abnormal (e.g. disconnect).

Bug: 36401174
Test: buffer_hub_queue-test passes.
Change-Id: Ifef5f737a5e737b70d19bdbffd7544a993438e1c