Bug 870171 - Part 14: Add PrunePolicy skeletion.
Bug 870171 - Part 15: Add attemptPruneByDuration.
Bug 870171 - Part 16: Add attemptPruneBySize.
Bug 870171 - Part 17: Add pruneEvents and tests.
Bug 870171 - Part 18: Add pruneEnvironments and tests.
Bug 870171 - Part 19: Add attemptVacuum.
Bug 870171 - Part 20: Have BroadcastService start PruneService alarm.
Bug 870171 - Followup: Put deletion inside transactions, add final keyword.
Bug 870171 - Followup: Take onCreate/onUpgrade out of a transaction.

While not strictly necessary since transactions can be nested, these
methods are contained in a transaction by default so pulling them out
should increase performance.
Bug 870171 - Followup: Put event table deletions into a transaction.