Delay updating of usage stats package mappings.

Do not update package mappings for the system user when the user is
first unlocked. Instead, schedule a job to be executed after 24 to 48
hours from when the system user is unlocked. This makes the service
initialization phase for the system user a little quicker since their
data is not likely to be stale. Additionally, this also ensures that
restored data is not pruned by mistake if there is a device restart
before restore is completed. The updating of the mappings occurs
normally for other users, on user service initialization.

Bug: 155209652
Test: manually ensure job is skipped for system user
Test: atest android.app.usage.cts.UsageStatsTest
Change-Id: I2c03a1a05246d6b454569c4569813e90bede3693