WifiManager: Return dummy values when wifi service is not up

Many system/vendor modules assume that wifi service is up by the time
they run. This is no longer the case since wifi stack now starts a
little later in the bootup sequence (to support pluggable wifi stack
module for mainline). If any of these modules invoke a WifiManager
call,
a) Return an appropriate dummy value if the API returns a value
b) Raise a RemoteException if the API returns nothing (void).

Bug: 139492857
Bug: 137650946
Test: Verified that SUW & CNE receiver no longer crashes with the stack
traces in the linked bugs.
Also, verified that SUW still can connect to wifi networks & detect when
wifi auto-connects after SUW comes up.
Test: Verified wifi/softap toggle from Settings/Sysui.

Change-Id: Ia100a4612fc903ae470a0c89523d4fda27eb3fbc