Limit the number of notis an app can enqueue

In addition to the number they can post. Also decrease
the number of posts per second to 5, but allow finished
progress notifications through.

Fixes: 63173849
Fixes: 63167456
Test: runtests systemui-notification & simultaneously
downloading 5 files.
Change-Id: I0c8ea35b3d9d38b000ea5fe383515e1d5a26bac7