Making 2 flags read only since they are used on backup and restore

The flags are narrow_grid_restore and grid_migration_fix.

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: Ia6a439553c3c5098115144eae532342ce73be4f2