bufferhubd: Implement more DetachedBuffer logic

1/ Separate DetachedBuffer related logic into a dedicated subclass of
   BufferHubRPC. This actually is the right thing to do as it utilizes
   the PDX's client/service programming pattern better.
2/ Add IsValid() check for the DetachedBufferChannel object.
3/ Add BufferHubClient to handle general PDX operations.
4/ Add DetachedBuffer which composites a BufferHubClient.
5/ Fully functional logic of allocating a DetachedBuffer, converting it
   to a BufferHub-backed GraphicBuffer, then converting it back to a
   DetachedBuffer.

Bug: 38137191
Bug: 70046255
Bug: 70912269
Test: buffer_hub-test
Change-Id: I81bf9259cbbaeb29a6df2769363b5a03464e7864