Add ability to opt out of auto-value-parcel for a given @AutoValue class

[chris-tate]

It would be fantastic to have an annotation to suppress auto-value-parcel for a specific class, so we can write the marshaling code by hand while still using @AutoValue for everything else. Right now, just having auto-value-parcel in my project means that it is active for any @AutoValue class that implements Parcelable. In a couple of cases where implementing Parcelable is a requirement this means I have had to abandon @AutoValue for those classes entirely because of the known issues around the way AutoValue subclasses versus the way AIDL generates marshaling code.