Persist Wifi frequency band

Wifi frequency band value was not being stored in system preferences,
since WifiService:setFrequency(int band, boolean persist) is never
actually called.

Change-Id: Ib70d97bed67fea4f06b80328158a135eb86cead7
Restore list preferences from system preference instead of currently used values

WiFi country code and frequency band are updated only when wifi is restarted.
Restore values to the list preferences from Settings.Global to convince the
user changes have been acknowledged.
Also make sure they are applied at every preference activity start.

Change-Id: I3c4e78a210550e5bc0b12d43a4f952dfd0035190