Managed Mode Updates

[garthvh]

Right now the managed mode is a protobufs only config option that is honored by the two phone clients.

• Allow updating of managed mode devices only over the admin channel, disable serial and phone api configuration changes.
• Client apps should check for an admin channel or shared key before enabling managed mode.
• Direct users to use the web flasher to wipe a device if they get into managed mode if they get stuck and can't turn it off.