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Introduce SystemFacade, an interface that allows us to stub out the
system clock for testing the download manager.  This allows us to test
retrying a failed download without having the test wait 60 seconds.
This interface can include other dependencies in the future as well.

I've also used this to add tests for 503 (retry-after) and 301
(redirect), and I've added a test for download to the cache partition.

Other changes:
* made MockWebServer capable of checking + rethrowing exceptions from child threads
* refactoring + cleanup of DownloadManagerFunctionalTest