Add Wifi calling Preference slice and other cleanup

WifiCallingPreference Slice Provider:
  1. If Wifi calling is not enabled - display message to user
     to enable Wifi calling
  2. Else if Wifi preference is not editable - return a null
     Slice
  3. Else provide a slice with rows - first row giving current pref
     information, followed by preference items (in each row).
     wifi_only is displayed only when it is allowed to display it.
     Each row with preference item has specific intent action.
WifiCallingPreference Slice BroadcastReceiver:
   1. If only Wifi calling is enabled & Wifi preference is editable &
      there is a change in current value modify wifi pref setting
   2. And Ask to re-query the slice in one second to display
      updated settings if 1 is valid or display appropriate message.
Clean-up:
   1. Return null instead of non-actionable slices.
   2. Use getText to get string resources.
   3. Remove unnecessary extra variables.

Bug: 79548264
Test: Use support-slices-demos-debug.apk to test on device
Change-Id: I186f19be2007c2331eaf6195e70b4a9c635adf9e