Fix P/Invoke signatures for the resource updater.

Port dotnet/sdk#3310 by @peterhuene

The resource updater was not specifying a CharSet for the Windows API
P/Invoke signatures and therefore the ANSI versions of the APIs were
being used.

This caused an unhandled exception when customizing the apphost on
Windows when the intermediate apphost path contained Unicode characters
because the ANSI resource updating API couldn't find the file.

Fixes ABdotnet/core-setup#900429.


Commit migrated from dotnet/core-setup@0059721

Author: William Li

src/installer/managed/Microsoft.NET.HostModel/AppHost/ResourceUpdater.cs

@@ -21,7 +21,7 @@ private sealed class Kernel32
             // Native methods for updating resources
             //
 
-            [DllImport(nameof(Kernel32), SetLastError=true)]
+            [DllImport(nameof(Kernel32), CharSet = CharSet.Unicode, SetLastError=true)]
             public static extern SafeUpdateHandle BeginUpdateResource(string pFileName,
                                                                       [MarshalAs(UnmanagedType.Bool)]bool bDeleteExistingResources);
 
@@ -71,7 +71,7 @@ public enum LoadLibraryFlags : uint
                 LOAD_LIBRARY_AS_IMAGE_RESOURCE = 0x00000020
             }
 
-            [DllImport(nameof(Kernel32), SetLastError=true)]
+            [DllImport(nameof(Kernel32), CharSet = CharSet.Unicode, SetLastError=true)]
             public static extern IntPtr LoadLibraryEx(string lpFileName,
                                                       IntPtr hReservedNull,
                                                       LoadLibraryFlags dwFlags);