IOMX: separate node interface from IOMX

- Separate node interface from IOMX. Client allocates the
  node through IOMX, and once it has the node interface,
  it can interact with the node directly. There is no need
  to go through IOMX and refer to the node by id.

- Simplify OMX, remove id-to-instance mapping on server
  side, and move the callback dispatch into node instance
  (as dispatcher is per-node).

- Keep track of node generation on client side to ensure
  that message handling ignores messages directed at an
  expired node.

- Remove MuxOMX, always use IOMX from media.codec.

- Minor whitespace reformatting.

bug: 31399200

Change-Id: I8aef6ec1fa5aba861da7d121960ed2974a963d81