LineageDatabaseHelper: Override onDowngrade()

* We had previously bumped the DB_VERSION to:
  - 14 (which set STATUS_BAR_CLOCK to 1 and STATUS_BAR_BATTERY_STYLE to 2)
  - 15 (which set STATUS_BAR_CLOCK to 2 and STATUS_BAR_BATTERY_STYLE to 0)
    DB_VERSION 15 is essentially reverting to version 13, and undoing the changes that version 14 did.

* Our R based builds follow upstream (which bumped the DB_VERSION to 14, from 13 on lineage-17.1)
  This breaks boot, for a user trying to transition with a Q userdata to R.

* At this point in time, it makes sense for us to force downgrade the DB_VERSION to 13, so that our upgrade path is left intact and we can follow upstream from R as well.