am df71cabb: cherrypick Change-Id: Ica1b68a1687917432d39d7d116c9c24fa1644a83 Initial posting of in-app billing docs

* commit 'df71cabbc2f670a4acea91e04f54990a472a56e9':
  cherrypick Change-Id: Ica1b68a1687917432d39d7d116c9c24fa1644a83 Initial posting of in-app billing docs