* Disable AndroidUseLatestPlatformSdk in VS so VS stops auto-updating Android projects;
Remove XF.targets imports from projects which don't need it;
Make XF.targets imports conditional on existence of XFBT DLL in VS to avoid errors
* More consistent check for VS
* Apply nicer VS check logic to Xamarin.Forms.Xaml.UnitTests.csproj
* Fix missing "'"
* Re-add XF.targets imports to PagesGallery native projects
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