Make RouteListingPreference getter and callbacks public

This change exposes getRouteListingPreference(),
onRouteListingPreferenceChanged(), and other callback-related
methods.

This is a non-functional change.

Test: Presubmit.
Bug: 192657812
Bug: 281067101
Change-Id: Iea44d9eef42bfb0add775b7514430739bfe39c62
API-Coverage-Bug: 294073926