MoKeeHelper: Create DownloadNotifier, simplify DownloadReceiver logic.

Change-Id: I394228b7cc01e3887d4c93eddc28be69b0595553
MoKeeHelper: Create builder for ItemInfo. Simplify codebase.

Change-Id: I03960af146ef50440824658e70350c08516cec91
MoKeeHelper: Handle download complete in background thread.

  - Since a broadcast receiver's timeout is ~10s, we should avoid doing
  too many operations, and instead offload to an intentservice.

Change-Id: Ic7a57a9fefd530d416df446d17c7d3e311aabc82