Prune UsageStats data belonging to uninstalled packages.

Add a per-user job to prune any usage stats data which belongs to a
recently removed package. This job will be scheduled when a package is
removed and it will be executed when the device is considered to be
idle. The jobs are persisted by job scheduler and they are also removed
when a user is removed since all usage stats data is deleted on user
removal.

When executed, the job reads all of the stats on disk and removes those
that belong to removed packages (packages which don't have a token
mapping). If no data is ommitted on read, a write is not performed. If
the user is in a locked state when the job is executed, the job will
keep getting rescheduled until a sucessful pruning of the data.

Additionally, add logic to prune any obsolete usage stats data on a
database upgrade, pruning all data belonging to packages which have
been uninstalled. This ensures that all data in UsageStats in R belongs
to packages that are currently installed or to packages whose
DONT_DELETE_DATA flag was set when uninstalling.

Also remove the clean-up mappings step on boot. That was added as a
safety measure to ensure the mappings file is always updated. However,
with the logic to prune on upgrade and on package uninstalls, that step
is now unnecessary.

This CL also refactors how the UserUsageStatsService is fetched and
initialized within UsageStatsService. This is to ensure there is no lock
contention when the user service is initialized and it also makes way
for other refactorings in the future related to the user service
initialization.

Bug: 143889121
Test: atest UsageStatsDatabaseTest
Test: atest android.app.usage.cts.UsageStatsTest
Change-Id: If475fc018a930d0956b85a64b4e34e2c75b2476f