[generator] Add 'managedOverride' metadata to allow user to force 'override'/'virtual'.

[jpobst]

Fixes: #681

There are scenarios where generator does not correctly find overridden methods, often due to generic parameters. When these issues happen, the fix is to <remove-node> the offending method and add a fully hand-bound version of it as an Addition. This is a very manual fix to simply add override to a method.

While we would like to eventually fix all these scenarios to "do the right thing", that will be a complex fix(es), and perhaps we will never be able to handle all cases.

For now, we can at least alleviate the pain by adding a metadata attribute that allows the user to override our detection, avoiding the hand-binding.

This new attribute is called managedOverride and allows the user to specify the values virtual or override, forcing us to use the requested modifier instead of generator's usual logic.

<attr path="//method[@name='example']" name="managedOverride">override</attr>
<attr path="//method[@name='example']" name="managedOverride">virtual</attr>

[jonpryor]

This <class/> is very odd -- and apparently something that ParseApiDefinition() doesn't check for -- in that [@name='Object'] is not consistent with [@jni-signature='Ljava/lang/EmptyOverrideClass;']. With [@name='Object'], I would expect [@jni-signature='Ljava/lang/Object;'].

[jpobst]

Looks like we don't even read the jni-signature attribute, much less do anything with it: https://github.com/xamarin/java.interop/blob/master/tools/generator/Java.Interop.Tools.Generator.Importers/XmlApiImporter.cs.