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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:5ea61960ab4b0da02aeb5dd36968c2ef923709aa)
Merged-In: Ia6a439553c3c5098115144eae532342ce73be4f2
Change-Id: Ia6a439553c3c5098115144eae532342ce73be4f2