We claim the ornamental design for handheld controller, as shown and described.
FIG. 1 is a front view of the handheld controller of the claimed design;
FIG. 2 is a left side view thereof;
FIG. 3 is a right side view of the claimed design;
FIG. 4 is a back view thereof;
FIG. 5 is a top view thereof; and,
FIG. 6 is a bottom view thereof.
The dot-dash lines in FIGS. 1 and 5 represent lines that form no part of the claimed design, and the dashed lines in FIGS. 1-4 and 6 are included for the purpose of illustrating structural features that form no part of the claimed design.