Use layer_name for thumbnail generation instead of hardcoded value.

[JoseCovaOS]

Changelog Description

layer_name is now being used for thumbnail generation instead of rgb hardcoded value. This allows thumbnail generation in Hiero versions below and above 16.

Testing notes:

1. Launch Hiero (a version below than 16).
2. Publish a plate (before, make sure plate products are configured to create an ftrack AsserVersion).
3. Check ftrack, created version should have a thumbnail.