If the user only had the default db, then migrate to the new default

We also check if the new default is bigger than or equal to the
previous one.

Flag: ACONFIG enable_grid_migration_fix enabled
Flag: ACONFIG enable_narrow_grid_restore enabled
Bug: 325286145
Bug: 325285743
Test: BackupAndRestoreDBSelectionTest.kt
Test: GridMigrationTest.kt
Change-Id: I98315a4e4c112ca96a74200dd76016803dfaf820