Rewriting "Audio and Video" section of Dev Guide.

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