Refactor grid migration

In this CL:
-Make GridsizeMigrationLogic functions pure (no side effects)
-Make it so GridSizeMigrationLogic contains the migration logic and does NOT interact with the database
-Let GridSizeMigrationUtil receive the results from the logic layer and use those results to interact with the database appropriately

In future CLs:
-Refactor GridSizeMigrationLogic to Kotlin

Bug: 358399271
Flag: com.android.launcher3.grid_migration_refactor
Test: GridSizeMigrationTest
Change-Id: Ifcc5ca356558a7463352580f2e89eef8c86ed16b