Replace instance field with read-only Instance property on singletons

[ShiJbey]

GameSession, PlayerProfile, GameWorld, StepManager, and FragmentManager are all singleton classes that need their instance fields replaced with read-only properties to prevent unintentional overwriting.

For example, in StepManager, we should do the following:

// Replace
public static StepManager instance = null;

// With
public static StepManager Instance { get; private set; }