Pairing mode for multiple devices on Android

[TroyNeubauer]

We need a way to bind different USB devices to multiple SDR sinks / sources. The workflow could look like this:

1. At flowgraph authorship time we will enforce that flowgraphs with multiple sources / sinks must name them appropriately
   At runtime:
2. When loading / running a flowgraph (maybe we need a pairing mode to allow the user to select when to do this action, and allow them to avoid re-pairing if the past selection is acceptable)

• For each source / sink:
  • Tell the user to plug in block XXX (grabbing the name the author wrote for that block)
  • Wait for USB callback from Android
  • Do some basic checks (is this an SDR, does seify detect this)

3. Pass this pairing of FD + block name / block ID to natives

• TBD on the interface for this. How do we identify a block across the FFI layer?
• Perhaps we have a native method that parses the toml and returns a Vec<(String, usize)> containing the (name, id) for each block
• When going to run the flowgraph we map this to a Vec<(usize, OwnedFd)> which is then used by #138