[xamlc] fix compiled {OnPlatform}

[jonathanpeppers]

A customer's project was doing:

{OnPlatform foo, iOS=bar}

When compiling with dotnet build -f net6.0-android -c Release.

The value foo was not set!

However, either of these combinations work fine:

{OnPlatform Default=foo, iOS=bar}
{OnPlatform Android=foo, iOS=bar}

It appears we are missing the case for the [ContentProperty] for the {OnPlatform} markup extension.

I added tests cases for this, and solved the problem by using node.CollectionItems[0] to get the actual value.

I think this is the right approach?

Issues Fixed

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1584968

[PureWeen]

/azp run

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