This change modifies `DownloadHelper` to check if a URL is already
scheduled for download (in pending, running, or paused status) before
enqueuing a new request. If a download for the same URL is found, the
existing download ID is returned, avoiding redundant requests to the
`DownloadManager`.
Bug: 437217681
Test: atest NetworkSecurityUnitTests
Flag: EXEMPT BUGFIX
Change-Id: I64dd5473b4fdb23241f5e2503c65cb64db56617c
b7c4c84a37