Resume uploads on different executions

If I execute a resumable upload for a file, kill the process and then start it again, **there is no way to get the "upload_id"** for the first file in order to continue with the upload in a new process. This can be tested with the `DriveSample` class on GitHub samples.

The class `MediaHttpUploader` creates and maintains a `currentRequest` (`uploadType=resumable&upload_id=...`) during one execution of `resumableUpload(GenericUrl)`. However, there is no way to obtain and maintain that information for future processes.

My intention is to upload backup files in the background. If the user closes my application, I should be able to continue with the upload when the user starts the application again.

Original post: [Issue 4131](https://code.google.com/a/google.com/p/apps-api-issues/issues/detail?id=4131).
Similar question [here](http://stackoverflow.com/questions/25288849/resumable-uploads-google-drive-sdk-for-android-or-java).

Start with the DriveSample class and MediaHttpUploader, then trace resumableUpload(GenericUrl) and its currentRequest containing the upload_id. Determine how the upload identifier can be obtained and maintained across processes; done means a later execution can continue the interrupted upload.

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