* [generator] Fix bug 42742 The advice attribute doesn't make it to the generated code
https://bugzilla.xamarin.com/show_bug.cgi?id=42742
The Advice attribute used in our bindings was not making
into the generated code. This fixes it.
Also added unit test for it
* [generator] Merged PrintFooAttributes into one
Merged PrintPlatformAttributes, PrintPreserveAttribute and
PrintAdviceAttribute into a single method PrintAttributes.
This is due to sometimes we call three times in a row the same method:
```csharp
PrintPlatformAttributes (pi.GetGetMethod ());
PrintPreserveAttribute (pi.GetGetMethod ());
PrintAdviceAttribute (pi.GetGetMethod ());
```
We now do
```csharp
PrintAttributes (pi, platform:true, preserve:true, advice:true);
```
Also removed a not used instance (internal field) of AdviceAttribute
Alex Soto