[Xamarin.Android.Build.Tasks] Fix an issue where library projects can not be debugged.

[dellis1972]

The linker was not producing ppdb files.
What is supposed to happen is the
linker if it changes a dll should emit a new pdb file
along with it. Because none of the assemblies were being
loaded with debug symbols. Hence no updated pdb.

So this commit loads the Symbols as part of the
linker process to ensure that we are able to emit the new
pdb when required. This is only done for files which
have been changed by the linker.

[radekdoulik]

I wonder what is the performance penalty of this change. Could you please try if not reading the symbols (as before) and adding following change would fix it? It might help to keep the performance nearly the same.

index b69dd41..61dbcd0 100644
--- a/src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/FixAbstractMethodsStep.cs
+++ b/src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/FixAbstractMethodsStep.cs
@@ -28,6 +28,7 @@ namespace MonoDroid.Tuner
                        }
 
                        if (changed) {
+                               Context.SafeReadSymbols (assembly);
                                AssemblyAction action = Annotations.HasAction (assembly) ? Annotations.GetAction (assembly) : AssemblyAction.Skip;
                                if (action == AssemblyAction.Skip || action == AssemblyAction.Copy || action == AssemblyAction.Delete)
                                        Annotations.SetAction (assembly, AssemblyAction.Save);

[dellis1972]

@radekdoulik ok I'll give that a go

[dellis1972]

@radekdoulik your suggestion seemed to work. Well my new unit test still passed so it was doing the same thing

[radekdoulik]

That's great, thanks!