The ornamental design for air treatment device, as shown and described.
FIG. 1 is a perspective view of the air treatment device embodying the new design:
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left-side view thereof;
FIG. 5 is a right-side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
FIGS. 1-7 may show features of the air treatment device in broken lines. The broken lines depict portions of the air treatment device that form no part of the claimed design. Further, the broken lines immediately adjacent the solid-line portions of the design form the boundary of the design, with the broken lines forming no part of the claimed design.