video device: use device id instead of friendly name

- Video settings, and APIs will now use a device id, which will be
  a path on platforms where everything is a file, and a DevicePath
  with a bonus ffmpeg-dshow compliant prefix on Windows.

- The device's friendly name is uniquified, and stored in the
  settings still, but only retrieved/translated for UI.

- MRLs are now constructed with the device id.

Change-Id: I092f08cc2cd31bd78aeec5c774c2cc33d75c1d4e