Improvements to Java API handling of datatypes and arrays (#12928)

* This is a set of changes broken out from #11535 (Generic Java API) and improved
a bit further; still synergistic with that PR.

* Changed how datatypes are computed to crawl over the array.

* Added the notion that a given array component type can correspond to more than one
  tensor datatype. We already would like this to be able to support both uint8 and string
  using byte arrays; the need will become more acute as more TensorFlow types are added.

* Resurrected the test case for creating and extracting uint8 tensors.

* Addressed Asim's comments (thanks!).

- Renamed class_datatypes and made it private.
- Changed dataTypeOf to walk the class objects instead of the class name. Seems clearer.
- Ran the code formatter.