Do not retain UsageStats for uninstalled packages.

When packages are removed, remove their associated token mappings in the
packages token data stored in UsageStats. When data is being persisted
to disk, the data for removed packages will not be written to disk.
Additionally, when any of the query APIs are called, data that is read
from disk for which a mapping does not exist will not be returned.

The data deletion uses a lazy techinque to avoid heavy costs of reading
and writing all of the usage stats data on every package removal.

Bug: 135484470
Test: atest IntervalStatsTests
Test: atest UsageStatsDatabaseTest
Change-Id: Ie32d65b47f86071c6a814a8b21e4be060519e598