Add Downloads.Impl.COLUMN_MEDIASTORE_URI & DownloadColumns.Description.

Entries from DownloadProvider are added to MediaStore Downloads
collection. COLUMN_MEDIASTORE_URI will be used to track corresponding
entries in MediaProvider. We can't re-use COLUMN_MEDIAPROVIDER_URI
for this purpose because it is updateable by apps.

Bug: 120876251
Test: atest DownloadProviderTests
Test: atest cts/tests/app/src/android/app/cts/DownloadManagerTest.java
Test: atest MediaProviderTests
Test: atest cts/tests/tests/provider/src/android/provider/cts/MediaStore*
Change-Id: Ifd252c54f4ee739a31be2866896efac6696a088e