Update USSD API docs per API review comments.

Expand docs on failure return codes.
@hide the success return code which is only used internally.
Expand docs on the UssdResponseCallback class to make them more clear
what the format of the USSD response is, and how it might be consumed.
Fixed broken java doc.

Test: Make docs, inspect docs.
Bug: 35766890
Change-Id: I7a3c26c436c60372116c5b43cb45c513f4276b8c