Add Downloads collection in MediaProvider.

+ Added needed logic to contribute files to this
  downloads collection using MediaStore.PendingSession.

Bug: 120050687
Test: atest cts/tests/tests/provider/src/android/provider/cts/MediaStore*
Test: atest MediaProviderTests

Change-Id: Ic83731e3f54908614cea13a84f717252062428dc
2 files changed