backends/native: Also check touch-mode on object initialization

On a Surface Pro 2017, touch-mode is currently only detected correctly after detaching and attaching the Type Cover (detachable keyboard) once, it seems that has_external_keyboard is only set to the correct value after MetaSeatNative is initialized.

So fix that and call update_touch_mode() once again when the object is initialized and the has_external_keyboard and has_touchscreen properties have been finally updated.