Add Android Parcelable support

A couple of weeks ago I [forked](https://github.com/frankiesardo/android-auto-value) `auto-value` to add support for Android Parcelables. Now I discovered than one of the main reasons for the fork, namely running `auto-value` at compile time without including its dependencies inside the apk, can work with the original artefact in Android Studio with a combination of both `provided` scope and `android-apt` plugin.

Since that is a workable solution I see no reason why Parcelable support could not be merged inside this repository. Would you be interested in a PR with android as an optional dependency or is it somehow out of scope for this library?