Это из исходного кода Android SDK.
Как видите, у вас есть специальный конструктор для создания Spinner с указанным режимом, который вы хотите использовать.
Надеюсь, это поможет вам:)
/**
* Construct a new spinner with the given context's theme, the supplied attribute set,
* and default style. <code>mode</code> may be one of {@link #MODE_DIALOG} or
* {@link #MODE_DROPDOWN} and determines how the user will select choices from the spinner.
*
* @param context The Context the view is running in, through which it can
* access the current theme, resources, etc.
* @param attrs The attributes of the XML tag that is inflating the view.
* @param defStyle The default style to apply to this view. If 0, no style
* will be applied (beyond what is included in the theme). This may
* either be an attribute resource, whose value will be retrieved
* from the current theme, or an explicit style resource.
* @param mode Constant describing how the user will select choices from the spinner.
*
* @see #MODE_DIALOG
* @see #MODE_DROPDOWN
*/
public Spinner(Context context, AttributeSet attrs, int defStyle, int mode) {
super(context, attrs, defStyle);