The ornamental design for an armrest apparatus, as shown and described.
FIG. 1 is a front view of an arm rest apparatus;
FIG. 2 is a rear view thereof;
FIG. 3 is a bottom view thereof, the top view being a mirror-image;
FIG. 4 is a right-side view thereof;
FIG. 5 is a left-side view thereof;
FIG. 6 is a perspective view thereof;
FIG. 7 is a referential view showing the arm rest apparatus with respect to a user; and,
FIG. 8 is another referential view showing the arm rest apparatus with respect to a user's arm.
The broken lines depict environmental subject matter only and form no part of the claimed design. The dash-dot lines depict a boundary between claimed and unclaimed subject matter, and form no part of the claimed design.