Java: Tensor implementation refactoring and style tweaks.

- Push more of the common checks for the create() methods that operate
  on a java.nio.Buffer to the helper method
- Apply Google Java Style formatting
  (https://google.github.io/styleguide/javaguide.html)
  to Tensor.java and TensorTest.java
- Group methods: (1) All create() methods first, (2) All "property" accessors
  (numBytes etc.), (3) All "export" methods (copyTo, writeTo)
- Share implementations of the private static and public member numElements()
- Avoid wildcard imports
Change: 146400289