video device: add unique id field for platforms that use path

With v4l2, the serialized device attribute currently named "id"
corresponds to the member "input" of the DeviceParams structure which
is used as a path. Ideally, the attribute should be called "uid" in
DeviceParams, serialized accordingly, and "input" should hold the
path or string required to open the device.

On linux, the "uid" should be something like: VID+PID+serial.

Considering the current broken state of device settings in the config,
migration on linux should be done by simply wiping the existing
entries.

Change-Id: I539330d33fdc8fc9c395469b330b9ce96f963958