Tech Debt: Update rename classes, methods and add some java doc.

[kshirsagarps]

There are some classes we should rename to fit the 3.0 architecture and conventions.
Also, we should have sufficient java doc comments for the methods and follow common guidelines throughout the code.

Some of the findings that will help achieve above goal. It would also be good to read through the other parts of the code and have it updated.

InjectionTarget:

• InjectionTarget can be renamed to BindingTarget.
• No Java doc comment for the class, what is injection target? It's a DS but what data does it hold and why?

InjectionTargetUtil:

• Can be renamed to BindingTargetUtil.
• No java doc comment, java doc comments to build methods like getOrCreateTargetClass
• For almost all the methods in various classes have the argument sequence - (
  TypeElement element, Map<TypeElement, InjectionTarget> targetClassMap), we should follow the same sequence

InjectExtraUtil:

• Can be renamed to BindExtraUtil, maybe/not.
• No java doc comment, java doc comments to build methods like