Merge "docs: Rewrite of App Fundamentals.. Part 1. This introduces two new...

Merge "docs: Rewrite of App Fundamentals.. Part 1. This introduces two new docs: Activities   Provides a fundamental introduction to how activities work, including   the basics about layout, manifest entries, starting activies, getting results,   and the complete discussion about the lifecycle (which used to be in the   fundamentals.jd document under "Component Lifecycles"), but also now includes more   information about saving instance state (with new diagrams). Tasks and Back Stack   A large expansion of the material originally presented in the fundamentals.jd   document under "Activities and Tasks". This doc talks all about how tasks and   the back stack work. This content re-organization is important because fragments   can now become a part of the back stack (which is a task-oriented concept), thus,   it's important to separate the task ideas from being tied directly to activities only,   which is how tasks are currently discussed. However, fragments are not yet discussed   here in detail---that will come later when the fragment dev guide is introduced."