[generator] refactoring for methods

[jonathanpeppers]

In the same fashion that 7a96efe refactored fields, this commit aims at
refactoring methods. I also added various unit tests to add coverage
around the new TestMethod support type.

Refactoring in generator:

• Dropped the method "support" classes in favor of extension methods
  from ManagedExtensions and XmlExtensions. These existed due to C#
  single-inheritance getting in the way of the relationship between
  methods, properties, constructors, etc.
• Moved methods from Method and MethodBase to CodeGenerator and
  renamed them from Generate* to WriteMethod*. I tried to not modify
  the code in these methods other than to add a parameter for the incoming
  method.
• Output of generator should remain the same.
• *Method classes are now alot closer to POCOs

Other changes:

• Dropped usage of ContextTypes stack in XamarinAndroidCodeGenerator
• Added a base test class for JavaInteropCodeGeneratorTests and
  XamarinAndroidCodeGeneratorTests because there were several tests that
  were completely identical (due to CodeGenerator base class)
• Changed StreamWriter to TextWriter as needed
• Now up to 88 tests! Most new ones are ~1ms

Future plans (trying to keep this commit of reasonable size):

• The CodeGenerator base class is starting to get large. As
  refactoring continues, I would like to make a class for
  FieldGenerator, MethodGenerator, etc. and CodeGenerator would get
  properties named Fields and Methods that would return these child
  generator classes
• This is exactly what grendel did with his prototype, so we should
  adopt this improvement:
  https://github.com/grendello/JavaBindingGenerator/blob/fb8864d1842bdf0b8d45eb04b89ef2bd57de0014/src/Java.Interop.Bindings/Compiler/MethodCodeGenerator.cs