Make user explicitly set security type for tether network

This was made implicit which is confusing for the user so this CL
adds a preference to make the user manually choose their security
type.

Test: robotests
Fixes: 79435112
Change-Id: Ie78806e8952b52e1b7cd21f0b87c9d064acaff64
Merged-In: Ie78806e8952b52e1b7cd21f0b87c9d064acaff64