Correctly restrict UI for WifiSettings in Guest profile.

Makes sure the restricted user does not see Wifi Preferences
or Saved Networks activity. Also fixes a bug where the progress
bar is displayed starting the activity in Guest mode, and does not go
away. LinkablePreference is changed to make sure titles can be
multi-lined.

Bug: 35317523
Bug: 35318292
Test: to be done in b/35324506
Change-Id: I4a07a2ab3a83c3adaf87226969fd6f16e41c9419