[Fabric] Initial scrollbar implementation for ScrollViewComponentView

[acoates-ms]

Description

This adds an initial scrollbar implementation. This covers basic scrollbar usage for horizontal and vertical scrollbars.

Other minor fixes found / implemented as part of this:

• Implemented PointerRoutedEventArgs::GetCurrentPoint which previously only worked for tags === -1.
• Added a RoundedRectangle primitive to CompositionSwitcher. (I'd like to move the switch to use that, and remove the switch specific APIs from CompositionSwitcher at some point in the future).
• Fixed a crash in UIA IsEnabled handling when accessibilityState is set to null.
• Fixed a crash when shift+tabbing to move focus backwards in the tab loop.

NYI Features for future PRs:

• shy scrollbars (switching into touch indicators only when mouse not near the scrollbar).
• Pointer capture when dragging the scrollbar thumb using mouse input.

Screenshots

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