docs: Rewrite of App Fundamentals.. Part 3

This introduces two new docs:

Application Fundamentals:
  This is the homepage for the app fundamentals; it introduces
  the security sandbox, each of the component types, intents,
  the manifest file, and app resources.
Processes and Threads:
  Most of this document is from the original fundamentals.jd doc;
  it describes more about how an app's process runs on android
  and the different levels of process importance used to determine
  which processes the system can kill and how threads are and
  should be used in an application. I've added more detailed information
  about threads, however, including information about using AsyncTask.

These documents are orphaned for now. This is the last set of documents
required before I'll change the side navigation to adopt the new
app fundamentals and redirect the old fundamentals.jd file.

Change-Id: Ie0c6540a3a6d86780718e2918b8dffcda37d925c