Replace Keystore Migration flag with a new

read-only flag.

Bug: 353597412
Bug: 352733140
Flag: android.net.wifi.flags.legacy_keystore_to_wifi_blobstore_migration_read_only
Test: atest WifiMigrationTest
Change-Id: Ia9dc6d08bd5a49383670c140fbc20e80cd5d43b1