Make generated code more aligned with Google Java style.

- Blank line after opening a message class (but not an enum interface).
- Let all code blocks insert blank lines before themselves. This applies to
  'package' statement, all message classes, enum classes or constant groups,
  extensions, bitfields, proto fields (one block per field; i.e. accessors
  don't have blank lines among them), and basic MessageNano methods. In this
  case we don't need to guess what the next block is and create blank lines
  for it.
- Fixed some newline/indent errors.
- Only one SuppressWarnings("hiding") per file.

Change-Id: I865f52ad4fb6ea3b3a98b97ac9d78d19fc46c858