fix(sidenav): move @Input to getter/setter instead of private field

[alxhub]

This breaks AOT compile since the generated factory tries to set the private field.

I'm unsure of the ramifications of change detection setting opened via the setter, which will call toggle() where it didn't before - someone who understands the code should look at that.

[jelbourn]

Closing this since the fix is already captured in #1225

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