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When deleting a file from DownloadManager, we also reach over and
clean up any scanned MediaStore entries.  However, DownloadManager
clients may not hold the WRITE_EXTERNAL_STORAGE permission, such as
when they downloaded a file into their package-specific directories.

The safest fix for now is to clear the calling identity and always
clean up the MediaStore entries ourselves, since DownloadProvider
always holds the required storage permission.

Bug: 29777504
Change-Id: Iea8f5696410010807b118bb56e5b897c53f0e1fe