fix ProgressiveDownloader infinite loop

[Artemych]

We could get an infinite loop inside the ProgressiveDownloader as downloadRunnable(RunnableFutureTask) could cache the PriorityTooLowException and then we would loop forever until the ProgressiveDownloader would be cancelled.

Detailed explanation: Thread 1 - ProgressiveDownloader working thread Thread 2 - ExoPlayerImpl working thread

1. ProgressiveDownloader adds C.PRIORITY_DOWNLOAD (Thread 1)
2. ProgressiveDownloader proceeds with C.PRIORITY_DOWNLOAD (Thread 1)
3. ExoPlayerImpl add C.PRIORITY_PLAYBACK (Thread 2)
4. ProgressiveDownloader using PriorityDataSource trying proceedOrThrow and receives an PriorityTooLowException
5. RunnableFutureTask caches the PriorityTooLowException
6. ExoPlayerImpl removes C.PRIORITY_PLAYBACK
7. infinite loop as RunnableFutureTask on get check the cached error state.

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[tonihei]

Thanks for reporting! We interestingly already fixed a similar issue in https://github.com/roblav96/ExoPlayer/commit/18f7d85c91996c0b6109b2bdec6f8e5ed7a50e44, but only for external retry loops that call the download method again. The internal loop after a PriorityTooLowException has the same problem of course and needs to be fixed as well.