Question

why is the ability of apple snails to survive an increase in the salinity of water considered an adaptation

Answer 1

Apple snails inhabit a wide range of ecosystems from swamps, ditches and ponds to lakes and rivers. The majority of the species prefer lentic water above streaming water and only a few species have adapted to rivers with strong current.

Breathing through the siphon when submerged (Pomacea canaliculata).

The lung/gills combination in apple snails reflects their adaptation to habitats with oxygen poor water. This is often the case in swamps and shallow waters. The without thelung they would completely depend on their gills, which would decrease their ability to survive. 
Another advantage of air breathing in combination with a shell door (operculum) is the ability to survive periods of drought often common in these habitats during the dry season.