Update behavior of setDestinationInExternalPublicDir().

Allow apps to use DownloadManager for downloading files into
known public dirs other than "Download" but don't include
them as part of Downloads collection.

Bug: 132136431
Test: manual
Test: atest DownloadProviderTests
Test: atest cts/tests/app/src/android/app/cts/DownloadManagerTest.java
Test: atest cts/tests/app/DownloadManagerLegacyTest/src/android/app/cts/DownloadManagerLegacyTest.java
Test: atest cts/tests/app/DownloadManagerApi28Test/src/android/app/cts/DownloadManagerApi28Test.java
Change-Id: I059c0b19218e07c7813e23c6b2d414d0306434d5