Enabled Backup/Restore of Package UsageStatistics

Backing up UsageStatistics using a new API in
UsageStatsService like the Notifications backup.
The backup logic is in the same package as UsageStatsDatabase
while the BackupAgentHelper resides in the android package
alongside SystemBackupAgent.

Bug: 26179323
Change-Id: I022d85fbcd4abb763230bec6eea50a7e723b5152