Special handling when a db for one grid option is not setup yet

- Init KEY_MIGRATION_SRC_WORKSPACE_SIZE and KEY_MIGRATION_SRC_HOTSEAT_COUNT
- Load default workspace only when default db is created, not when peeking into dbs of other grid options during grid preview / migration

Fixes: 154184711
Test: run grid preview and migration right after a cleared cache Pixel Launcher
Change-Id: I86c7072b8c4a9da76e289c55ab440071f192fc38