fix(overlay): flexible overlay with push not handling scroll offset and position locking

[crisbeto]

• Fixes the position of flexible overlays with pushing enabled being thrown off once the user starts scrolling.
• Fixes flexible overlays with pushing not handling locked positioning. With these changes locked overlays will only be pushed when they're opened, whereas non-locked overlays will stay in the viewport, even when the user scrolls down.
• Fixes a potential issue due to a couple of variables being initialized together where one is set to zero and the other one is undefined.

Fixes #11365.

[crisbeto]

Marked as P2, because the current behavior ends up being a regression in the menu component.

[jelbourn]

LGTM

[ngbot]

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