Accumulate updates to notifications before archiving.

By only adding notifications to the archive when they are
removed we batch up multiple updates and only store the
final version. Some data is lost in this process, but we
save tons of memory storing otherwise redundant /
uninteresting data (e.g. each step of a download).

Change-Id: I008afefc1242bb7c433d45da2c36fcc626dd3706