Fix OpenMAX AL doc errors

dd:
 - Add introductory paragraph from highlights
 - fix typos

db:
 - refer to Android Supported Media Formats

Change-Id: I22cd4b738fb96c9f01bf6b39d8cfb7144942205a
gk:
 - accidentally omitted section Media player prefetch,
   copied from OpenSL ES with minor changes
 - update interface charts
4 files changed