Support double-quote SSID in WifiService.

+ push the double-quote handling down to framework.

wpa_supplicant keeps the ssid in a quoted string in the config file. However,
the UI currently needs to handle the quoted string which makes it difficult
to handle the SSID containing the quotes. The change will move the
supplicant-specific double-quote handling from UI to framework, i.e. to
add/remove doubel-quotes in framework instead of in UI settings.