Downloader specific parameters and constructor is moved to DownloadAction class.
DownloadAction objects need to be serialized so they can be passed in Intents and can be stored to
filesystem (to be implemented). So DownloadAction.Serializer is added. Didn't use Serializable interface
because of the concerns over incompabilities when the new versions of classes are used.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=164831115
