Don't assume all downloaded files are under top-level Download dir.

Apps could have downloaded files into their own package-owned paths
and those targeting P or lower, anywhere.

Fixes: 128320098
Test: manual
Test: atest DownloadProviderTests
Test: atest cts/tests/app/src/android/app/cts/DownloadManagerTest.java
Change-Id: I28d9f9b8017b66095fb1abeb9a389a18770989a4