Fix WifiInfo#getHiddenSSID

The method would previously always return false.

Note that this is the simplest fix, and not the most
optimal fix. That said, in this change, a call to
WifiInfo#getHiddenSSID costs the same as a call to
WifiInfo#getSSID, so it isn't that bad.

bug: 10705138
Change-Id: I980563fd8a5bafe8c58c586449e778a0aa8b58a4