Blender Git Loki

Fix `Action.asset_data["is_single_frame"]` set incorrectly

The asset metadata custom property `["is_single_frame"]` was set
incorrectly. Since this is intended for forward compatibility, including
being covered by the asset metadata indexing, it's important to have it
set correctly from the first release of Blender that includes the asset
browser.

Differential Revision: https://developer.blender.org/D13452