Rely on `AppHost` to more accurately track active gameplay time instead
of doing naive DateTime.UtcNow subtraction in UpdatePre/UpdatePostGame()
calls.
With this change, the play time reported internally by a game's gameplay
timer should match (or be much closer to matching) what Ryujinx displays
in the application library.
Aside from this being closer to the natural expectation of what "hours
played" would take into account (as by definition time spent paused is
time spent not playing), this also brings us closer to the behavior of
other emulators and game libraries.
