ipn: make Notify.Prefs be a *ipn.PrefsView

It is currently a `ipn.PrefsView` which means when we do a JSON roundtrip,
we go from an invalid Prefs to a valid one.

This makes it a pointer, which fixes the JSON roundtrip.

This was introduced in 0957bc5af2.

Signed-off-by: Maisem Ali <maisem@tailscale.com>
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