Revise the default IA rule of nested class - the default IA is same with their enclosing class

see https://lists.apache.org/thread.html/d05bfaa0134502a47f6e1aca56cb0b096d4dd32ddefbbdf28db4952a@%3Cdev.hbase.apache.org%3E for more discussion.

The related description is in https://hbase.apache.org/book.html#hbase.client.api.surface.

If a class is not annotated with one of these, it is assumed to be a InterfaceAudience.Private class.

We should add another rule for nested class - "nested classes take on the IA of their enclosing class."