Implement DetachedBuffer::Promote

1/ This enables a BufferHub-backed DetachedBuffer to be promoted into
   the ProducerBuffer.
2/ Add DetachedBuffer::IsConnected in addition to
   DetachedBuffer::IsValid.
3/ Cleaned up some using pdx:: namespace statements.

Bug: 69982239
Bug: 69981968
Bug: 70046255
Test: buffer_hub-test
Change-Id: I6ee99507b190d142647455532cdce0c2c780b2b0