Wifi: Introduce a configuration parameter to disable WiFi in ECBM

This configuration parameter aims to address the following requirement.
a) If the Wifi radio on the UE is in turned on state (and the UE is not
   connected to any WWAN) and the user dials 911 and the UE is not
   capable of making E911-VoIP over WiFi calls, then the UE shall turn
   off its WiFi radio and shall try to attach to one of the WWANs,
   depending upon the air interfaces supported by the UE  for setting
   up the 911 call to the relevant PSAP. After the 911 call ends,and
   after the callback period ends, then the UE shall turn on the WiFi radio.

b) If the Wifi radio on the UE is in turned on state and the UE is connected
   to a WWAN and the user dials 911 and the UE is not capable of making
   E911-VoIP over WiFi calls, then the UE shall turn off its WiFi radio and
   shall set up the 911 call over the WWAN to which it is already attached,
   if that WWAN is available and able to support 911 calling. If that WWAN
   is not available or is not capable of supporting 911 calling, then the UE
   shall select an available WWAN for setting up the 911 call. Assumption is
   that while a LTE network may not have 911 calling support capability, all
   1x, GSM and UMTS networks will be 911 capable. After the 911 call ends,
   and after the callback period ends, then the UE shall turn on the WiFi radio.

c) If the Wifi radio on the UE is in turned on state (and the UE is not connected
   to any WWAN) and the user dials 911 and the UE is capable of making E911-VoIP
   over WiFi calls, then the UE shall not turn off its WiFi radio but shall first
   try to attach to one of the WWANs, depending upon the air interfaces supported
   by the UE , for setting up the 911 call to the relevant PSAP.

d) If the Wifi radio on the UE is in turned on state (and the UE is also connected
   to a WWAN) and the user dials 911 and the UE is capable of making E911-VoIP over
   WiFi calls, then the UE shall not turn off its WiFi radio but shall first try to
   attach to one of the WWANs, depending upon the air interfaces supported by the
   UE,for setting up the 911 call to the relevant PSAP.

Thus,the following address the requirement above.
1) Introduce a parameter (KEY_CONFIG_WIFI_DISABLED_IN_ECBM) to conifgure Wifi disable
   in ECBM for the requirement c & d. This key shall be overridden in the specific
   carrier overlay configuration file (defaulted to FALSE).
2) Already existing API (setWifiEnabled()) to turn ON/OFF Wi-Fi cater the requirement a & b .

Bug: 27854016
Change-Id: I5af370c143630bdd4b075f4730fd1de1bbe1fe7d