SDK / exoplayer

1. Pull up all subclasses of DownloadAction into DownloadAction
2. Add DownloaderFactory for Downloader instantiation, and DefaultDownloaderFactory
   to replace the instantiation logic being removed from the DownloadAction
   subclasses.

This change will upgrade existing action files gracefully (i.e. it does not
lose compatibility with the existing offline implementation, other than some
minor breaking changes to the API).

TODOs:

1. Move test methods from the XDownloadActionTest classes into DownloadActionTest.
   This will be done in a subsequent CL. There's a lot of consolidation that can
   be done here, including de-duplicating some of the test code added in this CL.
2. Look at merging DownloaderConstructorHelper into DefaultDownloaderFactory.
3. Use customCacheKey in DASH/HLS/SS Downloaders, for completeness.

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