Changed public constants from integers to strings. Internally
everything is still integers, since we want that more efficient
representation for most things.
Changed the Callback interface to OnOpChangedListener. We also
have a private versin that again takes an int, and tricks to
make both work.
Reworked the class documentation to be appropriate to the SDK
(as much as it can be); most of the existing documentation is
moved to the private implementation. Also added documentation
of the MODE constants.
Change-Id: I4f7e73cc99fe66beff9194e960e072e2aa9458f8
9bb0ee9131