feat(ble): Use appearance set in the BT_DEVICE_APPEARANCE config (#3115)

Properly use the BT_DEVICE_APPEARANCE Zephyr symbol to advertise
the correct appearance, allowing overrides, for e.g. mice.

Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>

app/Kconfig

@@ -25,6 +25,9 @@ config USB_DEVICE_PID
 config USB_DEVICE_MANUFACTURER
     default "ZMK Project"
 
+config BT_DEVICE_APPEARANCE
+    default 961
+
 config BT_DIS_PNP_VID
     default 0x1D50