Changed AccessPointPreference setTitle to use getTitle()

AccessPointPreference now uses getTitle() as the single method
for the UI title. The saved network case is now also handled by
getTitle().

Marked getConfigName as deprecated in order to encourage use
of getTitle() as the definitive UI title for AccessPoints.

Test: atest AccessPointTest && atest WifiTrackerTest && atest
WifiSettingsTest.
Bug: 118705403

Change-Id: Ieaaef804e46f2eed2543e658aee030021b7d08df