Make ExceptionTypePolicy in NullPointerTester public

[gandhishrey]

NullPointerTester in guava-testlib has support for testing against different exception types based on a private enum ExceptionTypePolicy. However, this is currently set as a private field with no constructor or setter taking this as an input.

Setting an ExceptionTypePolicy or list of expected Throwables will be particularly useful for repos which throw IllegalArgumentException instead of NullPointerException for null inputs.