Add a callback for users of the StorageManager API to be able to receive
notifications when the requested operation completes for mountObb and
unmountObb.
Add NDK API to get to ObbInfo like the Java API has.
Also update the docs for the API and remove the "STOPSHIP" comments.
Change-Id: I23a4409c7f8b74d3169614beba920b4d667990a4