Fix resize frame corner radius for widgets added with button

Before this CL, the resize frame showed a mismatched corner radius
for widgets that have a non-default corner radius, when using the
add button. This is because widgets added with the button may not have
had `onLayout` called before the resize frame is set up.

This change adds a layout listener to the widget view, so that the frame
can set its corner radius once the view has been laid out.

Bug: none
Test: manual test
Flag: ACONFIG com.android.launcher3.enable_widget_tap_to_add NEXTFOOD
Change-Id: Ia9c37ae595be48e4ecab7930e17efca9a27ed05d