Update UsageStatsDatabase#KEEP_BACKUP_DIR.

Delete the backup usage stats directory on a successful upgrade to v5.
This was kept as true such that a revert to the previous version  would
be possible if needed.

Also indicate that an upgrade was performed such that relevant data is
pruned on dogfood builds.

Bug: 139937606
Test: manual: dir is deleted on migration.
Change-Id: I60285dc812a045bc82dd68a6e7ae87c2d3563adc