This fixes a bug where only the time sample for the first frame is written to a USD animation. Replaced the hard-coded USDExporterContext::time_code value with a USDExporterContext::get_time_code function wrapper which is called from USDAbstractWriter::get_export_time_code() to query the current frame when writing an animation. Pull Request: https://projects.blender.org/blender/blender/pulls/111248