Replace SCRCPY_ICON_PATH with SCRCPY_ICON_DIR

SCRCPY_ICON_PATH defined the path of the scrcpy app icon.
SCRCPY_ICON_DIR defines the directory where scrcpy icons reside.

This change prepares for the addition of other icons.

PR #6662 <https://github.com/Genymobile/scrcpy/pull/6662>