Fill ScrollView when content is smaller than viewport

[hartez]

Description of Change

When the content of MAUI ScrollViews is less than the size of the viewport and the content is set to Fill, the content remains at the small size rather than filling the viewport. These changes ensure that the content fills the viewport of the ScrollView on each platform.

This also moves several of the automated ScrollView unit tests to device tests (since differences in the platform ScrollView implementations mean that the actual layout of ScrollView content is platform-dependent).

Finally, this adds an actual vertical content alignment to the iOS Label implementation; the previous implementation (which attempted to align the content by adjusting the UILabel's Frame) caused complications and bugs with nested content (e.g., a Label with a Background inside a ScrollView).

Issues Fixed

Fixes #6058
Fixes #6694

[jsuarezruiz]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[mattleibow]

Looks good. I think this will require an update to the publicapi.txt, but testing to see how to do a remove.

-public override void PlatformArrange(Rect frame)
+public override Size GetDesiredSize(double widthConstraint, double heightConstraint)