Pass in dbFile from idpGridState for grid migration

- IDP can get re-initializeed during grid migration task, causing src/dest dbFile to be the same when prepareForMigration, which fails the migration and causes a wipe
- The fix is to cache IDP state as DeviceGridState, and use DeviceGridState to determine migration, trigger migration and save configuration after migration
- This should avoid migration failure in b/198965093 that causes homescreen wipe. Homescreen icon may still be messed up by the migration, but won't completely disappear

Bug: 198965093
Fix: 223579388
Test: manually trigger grid migration
Change-Id: I9890a6d5d530ff87aed8f601ec7d35a672b7cd46