Deprecate buffer_hub_client.{h, cpp}

buffer_hub_client.h used to contain BufferHubBuffer, BufferProducer
and BufferConsumer. Now those classes are all renamed and moved into
their own headers. Time to deprecate buffer_hub_client.h and move
BufferHubClient, which is only used by DetachedBuffer into
detached_buffer.h

Bug: 116855254
Test: atest buffer_hub-test
Change-Id: Id0bfb32165f955c060d982f1410ab11bfcf76bec