Java API Generics Phase 3. (#13421)

* Java API Generics Phase 3.

- Added the utility Tensors class.
- Updated tests to use it.
- Updated scripts for generating Tensors.java and the types directory.
  Note that these are still run manually, but remain helpful because
  maintaining so many methods and their documentation is a headache.
- Added missing checking for attempts to create tensors from arrays of boxed
  primitives, and a test case.

* Oops. This is the generated file!

* Addressed Asim's comments.
- made Tensors final
- got rid of unused state in UInt8 objects
- tuned up various javadoc comments