Fix(cdk/overlay): vertical positioning of flexible connected overlay on mobile

[ShukinDmitriy]

Fixes #13614.

Summary

Flexible connected overlays using overlayY: 'bottom' calculated their position
relative to documentElement.clientHeight, even when their actual containing
block was the overlay container with different dimensions.

This change calculates the bottom offset relative to the overlay container for
regular overlays. Native popovers continue to use the viewport because they are
rendered in the top layer and have a different containing block.

The fix does not measure scrollbars or create temporary DOM elements in the
reposition path.

Testing

Added regression coverage for:

• an upward-flowing overlay on a long, horizontally and vertically scrollable page;
• positioning before and after scrolling and calling updatePosition();
• a shifted and resized overlay container;
• native popovers whose containing block remains the viewport.

The CDK overlay unit suite passes in Chromium and Firefox.

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[ShukinDmitriy]

Hi! I’ve rebased and refreshed this PR against the current main.

The previous scrollbar-based implementation has been replaced with a
containing-block-aware calculation, and regression tests have been added for
the scrolled overlay-container path and the newer native popover path.

The CDK overlay unit suite passes in both Chromium and Firefox. This is ready
for review again. Thank you!