Handle AutoValue Extensions

[msridhar]

AutoValue has extensions that can affect which methods of an @AutoValue type correspond to properties. See, e.g, the following comment:

https://github.com/google/auto/blob/4543619f73dc94cc9a7232f340067e5bfe8ddb24/value/src/main/java/com/google/auto/value/processor/AutoValueProcessor.java#L193-L198

We need to find a way to make our AutoValue support robust to extensions "consuming" abstract methods like Parcelable.describeContents(). Right now, we get a crash because we see an abstract method like describeContents() but no corresponding setter in the builder.

/cc @lazaroclapp

[msridhar]

More details on why this is hard. AutoValue Extensions can "consume" arbitrary abstract methods of the @AutoValue class, so that AutoValue doesn't generate a concrete implementation of them:

https://github.com/google/auto/blob/389355d065fbe189d8f7a388beeaebeaaf1d13ae/value/src/main/java/com/google/auto/value/extension/AutoValueExtension.java#L383

If a method gets consumed, then there is no corresponding setter in the generated builder.

To know which methods are getting consumed in the current build, we need to know what each extension is doing. But, the extensions are run by AutoValue, which may run before or after our checker. My feeling is the best hope we have is to run the extensions ourselves to see what is getting consumed, but that would take some significant investigation / implementation, and it might not even work.