Use DefaultDllImportSearchPathsAttribute

[jonpryor]

Context: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1139578
Context: https://liquid.microsoft.com/Web/Object/Read/ms.security/Requirements/Microsoft.Security.SystemsADM.10039#guide

The current security guidance is that the System.Runtime.InteropServices.DefaultDllImportSearchPathsAttribute attribute should be placed either on the assembly or on [DllImport] methods, to control and constrain where LoadLibraryEx() will look for native libraries.

The advice is to use:

[assembly: DefaultDllImportSearchPathsAttribute(DllImportSearchPath.SafeDirectories)]