RescueParty: Add a DeviceConfig killswitch

1. Add a killswitch DeviceConfig flag in case emergency rollback is
needed (e.g. PackageWatchdog doesn't behave correctly).
2. Register the namespace for native flag sync mapper so that we can
also use the flag to turn off native disaster recovery.

Test: Atest RescueParty & verified on local device
Bug: 149420506
Change-Id: I9a0b546c7229c8444947b522585dd2c5ed37a00b