Rewrite of download notifications.

Switch to using new inbox-style notifications when collapsing multiple
downloads. Correctly handles clustering, including cancellation of
stale notifications. All notifications are now handled in a single
class, making it easier to reason about correctness.

Fixed bugs around handling of visibility flags. Move away from using
"int" as internal keys, since they can overflow. Started work for
time estimates, will finish in a future CL.

Explicitly pass all relevant IDs to DownloadReceiver instead of doing
a second racy query. Fix StrictMode warnings when querying in

Bug: 6777872, 5463678, 6663547, 6967346, 6634261, 5608365
Change-Id: I5eb47b73b90b6250acec2ce5bf8d7a274ed9d3a9
8 files changed