Inline unknownFieldData{Equals,HashCode} to generated code.

It turns out dex (apparently) was inlining these protected final
methods from ExtendableMessageNano into every message class. Removing
these methods from the base class and inlining their code reduces
the method count by 2 methods / message when the store_unknown_fields
option is on.

Change-Id: I0aa09f2016d39939c4c8b8219601793b8fab301f