Make toString(), hashCode(), equals() and friends final in AutoValue classes
Developers usually don't look at the classes that AutoValue generates,
but at the hand-written classes. If the hand-written classes implement
toString(), hashCode(), equals() etc, AutoValue doesn't override them.
To make this more clear to readers of the hand-written classes
ErrorProne suggests to make these methods final.
Enable this check at ERROR severity to prevent future occurrences.
Signed-off-by: Edwin Kempin <email@example.com>