DO NOT MERGE: Check that file exists in disk in queryChildDocs

Check that files returned from querying DownloadManager in
queryChildDocuments and queryChildDocumentsForManage still exist in
External Storage.

Bug: 28430547, 31108493

Change-Id: I4efe9f9ba79d41b3c3099fb96d18460880b742b5
(cherry picked from commit c45f4998c0b4f7ab44d3d2a180ca977ca2fa5ca8)
(cherry picked from commit 314026d4e85d0d1d075d5de83038cec1e751ec64)
1 file changed