Use @exportSymbol instead of @exportClass|Function

This commit uses @exportSymbol in place of @exportClass and @exportFunction for exporting single-arg constructors and functions. This represents a major simplification, removing the need for the specific @exportClass and @exportFunction statements, and the corresponding Export constructors and functions doing "external objects to internal objects" translations in the generated exports.js file.

This simplification relies on external @interface's (defined in the generated externs file) preventing renaming of properties. See this closure-compiler-discuss thread for more details: https://groups.google.com/forum/#!topic/closure-compiler-discuss/G3B9oSN6a4c.

This change removing the use of @exportClass and @exportFunction may allow using the @export annotation in the future.

With this change, and @exportClass and @exportFunction not being used anymore, the generate-exports.py script can be greatly simplified. This will be done in a future commit if the simplified mechanism works as expected.