[2D][Sprite Editor] Instantiated default assets can be edited in sprite editor

Instantiated default assets can be editted in sprite editor but no changes are saved

Repro steps:
- Open project > Open SampleScene
- Select Square in scene and open Sprite Editor window
- Observe square sprite can still be manipulated (edit sprite rect, edit custom outline and create bones/skinning)

Expected results: Sprites with references to default assets in package should not be editable
Actual results: Sprites with references to default assets in package are editable