Define "Add network" preference in XML.

Move the definition from Java to XML, so that SearchIndexProvider can
include it in the getXmlResourcesToIndex().
Also update getNonIndexableKeys() to return the corresponding key when
the preference is hidden.

If the Internet settings are restricted, "Add network" still appears on
search. When clicked, it opens the Internet settings page in restricted
mode, so that users can know that the settings are restricted.

Bug: 224421762
Fix: 224421762
Test: atest SettingsRoboTests:NetworkProviderSettingsTest
Change-Id: Ie9ea58070843a390844b461ce766f0ce29e64fab