Removing dead code from Launcher provider

> Removing DB upgrade logic for versions before 12. Launcher3
was forked off Launcher2 when DB version was already 12, so, there
can never be a lower version of the DB.
> Removing logic for auto import of launcher2 DB. This flow had
been permanently disabled by a flag, and we show a confirmation
prompt/cling before migration

Change-Id: I2ac2f21fefc41ee247164566a088927263b09459