3.窗口层级_层级结构树添加窗口

尚未添加窗口的层级结构树，如图
DisplayArea层级结构中的每一个DisplayArea，都包含着一个层级值范围，这个层级值范围表明了这个DisplayArea可以容纳哪些类型的窗口。
每种窗口类型，都可以通过WindowManagerPolicy.getWindowLayerFromTypeLw方法，返回一个相应的层级值。

/**
     * Returns the layer assignment for the window type. Allows you to control how different
     * kinds of windows are ordered on-screen.
     *
     * @param type The type of window being assigned.
     * @param canAddInternalSystemWindow If the owner window associated with the type we are
     *        evaluating can add internal system windows. I.e they have
     *        {@link Manifest.permission#INTERNAL_SYSTEM_WINDOW}. If true, alert window
     *        types {@link android.view.WindowManager.LayoutParams#isSystemAlertWindowType(int)}
     *        can be assigned layers greater than the layer for
     *        {@link android.view.WindowManager.LayoutParams#TYPE_APPLICATION_OVERLAY} Else, their
     *        layers would be lesser.
     * @return int An arbitrary integer used to order windows, with lower numbers below higher ones.
     */
    default int getWindowLayerFromTypeLw(int type, boolean canAddInternalSystemWindow) {
        return getWindowLayerFromTypeLw(type, canAddInternalSystemWindow,
                false /* roundedCornerOverlay */);
    }

    /**
     * Returns the layer assignment for the window type. Allows you to control how different
     * kinds of windows are ordered on-screen.
     *
     * @param type The type of window being assigned.
     * @param canAddInternalSystemWindow If the owner window associated with the type we are
     *        evaluating can add internal system windows. I.e they have
     *        {@link Manifest.permission#INTERNAL_SYSTEM_WINDOW}. If true, alert window
     *        types {@link android.view.WindowManager.LayoutParams#isSystemAlertWindowType(int)}
     *        can be assigned layers greater than the layer for
     *        {@link android.view.WindowManager.LayoutParams#TYPE_APPLICATION_OVERLAY} Else, their
     *        layers would be lesser.
     * @param roundedCornerOverlay