Rewriting "Audio and Video" section of Dev Guide.

Rewriting the "Audio and Video" section of the Dev Guide, renaming it
Media Framework and adding a more details about MediaPlayer,
interaction between MediaPlayer and services, Audio Focus, etc.

Staging:
http://www.corp.google.com/~btco/tmp/offline-sdk/guide/topics/media/index.html

This is a cherrypick of CL 106666 which was originally on honeycomb-mr1.
Link to old CL:
https://android-git.corp.google.com/g/#change,106666

Change-Id: I0efa4c9ed73af7f9dcb02232a6af4a7527645367