Fix handling of hidden access points

We now get raw hex data from the supplicant and we convert it into
printable format.

For hidden access point, we always used to return a single empty string.
We need to make sure we maintain that behavior for apps to not start
displaying empty strings.

Bug: 7310749
Change-Id: I2599b9b5e15be91fc34e9af629ad893b1a0357fc