Enemy (AI) Gremlins always use slow walk animations, even though their owner is in red alert

`XGUnitNativeBase.ShouldUseWalkAnim()` only checks if the unit is AI and on green alert. Gremlins as cosmetic units don't have gameplay interaction to allow alert levels, so they always use the walk anim -- which is the source of slowdowns in mods that use enemy gremlins.

The function should check whether the unit is cosmetic, and if so, if the owner is in green alert.