Restore prefab state after instantiation

[cooloon]

Instantiated prefab itself (not clone in the scene) remains inactive even after play mode.
This PR restore prefab state just after instantiation.

If that's not inteded, just drop this PR 😃

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[hadashiA]

Oh.. ok. You are right, it's not good that the prefab remains inactive.

I would like to add a little work to it.

• Add some tests
• I'd like to reduce the number of calls to SetActive a bit (since the callback in the hierarchy is called)

However, I would like to merge this. Thanks!