high-frequency notification stats.

Aggregate and then periodically report stats that are high-frequency
because they are driven by app behavior, not user behavior.

Reuse the NotificationUsageStats facility.
Remove redundant stats.
Lessen memory foot print.
Enable in-memeory aggregates with small, bounded memory footprint.

Bug: 20258744
Change-Id: I87e391419c53917fa13c68a56f8cdb40a7c8e548