[Xamarin.Android.Build.Tasks] Fix APT2264 error on Windows. (#8289)

We see users reporting the following error on Windows machines.

	APT2000 The system cannot find the file specified. (2): foo.xml

This is odd because what it *should* be reporting an APT2264 error.
This provides additional feedback to check for Long File Paths.  As a
result users do not get the additional info.

It turns out that `aapt2` generates subtly different error messages
between Linux/macOS and Windows, replacing a `.` with `:`.

  * Linux/macOS:

        The system cannot find the file specified. (2).

  * Windows
        
        The system cannot find the file specified. (2):

Because our APT2264-checking code included the trailing `.` and
Windows instead had a trailing `:`, we would *never* return APT2264
on Windows.

Lets fix that by removing the trailing `.` from what we look for.

src/Xamarin.Android.Build.Tasks/Tasks/Aapt2.cs

@@ -493,7 +493,7 @@ static string GetErrorCode (string message)
 			Tuple.Create ("APT2261", "file failed to compile"),
 			Tuple.Create ("APT2262", "unexpected element <activity> found in <manifest>"),
 			Tuple.Create ("APT2263", "found in <manifest>"),  // unexpected element <xxxxx> found in <manifest>
-			Tuple.Create ("APT2264", "The system cannot find the file specified. (2).") // Windows Long Path error from aapt2
+			Tuple.Create ("APT2264", "The system cannot find the file specified. (2)") // Windows Long Path error from aapt2
 		};
 	}
 }