Add new DroidDoc/JavaStub parser for parameter names retrieval.

[atsushieno]

These tools are used for new parameter names fixup in xamarin-android
so that we can test generator sanity for Mono.Android.dll and also that
we don't have to check in api-*.xml.in anymore.

[jonpryor]

Would it make sense/be possible to "better integrate" these utilities?

For example:

• build-tools/xamarin-android-docimporter-ng/xamarin-android-docimporter-ng/ would be more consistently added as build-tools/xamarin-android-docimporter-ng/ (removing the "intermediate" xamarin-android-docimporter-ng/ path)
• build-tools/xamarin-android-docimporter-ng/Xamarin.Android.ApiTools.ParameterNameExtractor would be more consistently added as src/Xamarin.Android.ApiTools.ParameterNameExtractor.

I suspect that the reason for this directory layout is to simplify future merging/parallel development in atsushieno/xamarin-android-docimporter-ng. If that's the case, and you want to keep them separate, perhaps it would be more sensible to use lib/xamarin-android-docimporter-ng?

[atsushieno]

Moving directories around is doable, but the reason why they are put under build-tools is because they are build-time-only tools. And having build-tools/Xamarin.Android.ApiTools.ParameterNamesExtractor and build-tools/xamarin-android-docimporter-ng is not very helpful to understand the relationship between those two. I don't see the point of changing current directory structure.