I claim the ornamental design for a mobile device holder, as shown and described.
FIG. 1 is perspective view of a mobile device holder, showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The dash-dash broken lines in the drawings illustrate environmental structure which form no part of the claimed design. The dot-dash broken lines define the bounds of the claimed design and form not part thereof.