Disable KeyBindingManager on right Alt keydown.

This fixes issue #8666.
I'm concerned about using `_enabled` to control this process since `setEnabled()` is a public API and you don't have full control over how it's turned on/off. I don't see this API be called anywhere in Brackets core code, so I'm curious as to how it's used.

Maybe the worst is that event handlers could leak, but can a new flag should be used for this?