Fix the end-icon missing issue for Wi-Fi networks

- Because WifiEntryPreference needs to support policy transparency, the inherited class is changed from Preference to RestrictedPreference.

- The RestrictedPreference will hides the end icon as default, causing this issue to happen.

- Override the getSecondTargetResId() function with necessary resource id to avoid the end-icon being hidden.

- Remove the redundant setWidgetLayoutResource, because it will be initialized in the extended class.

Bug: 262456749
Test: manual test
make RunSettingsRoboTests ROBOTEST_FILTER=WifiEntryPreferenceTest

Change-Id: Ie3d3a649d3137dc705d7887c8e74592437d9ce61