Class initializers prevent ProGuard from inlining any methods
because it thinks the class initializer may have side effects.
This is true for static methods, but instance methods can still
be inlined, because to have an instance you will have touched
the class and any class initializers would have run. But
ProGuard only starts inlining instance methods of classes with
class initializers from v4.11b6, and Android uses v4.4 now.

This change tries to avoid the class initializers as much as
possible, by delaying the initialization of the empty array and
some fields' saved defaults until when they're needed. However,
if the message hosts any extensions, they must be public static
final and therefore introducing the class initializer. In that
case we won't bother with lazy initialization.

Change-Id: I00d8296f6eb0023112b93ee135cdb28dbd52b0b8