Update widgetsModel to return pickable vs all widgets separately.

Earlier wallpaper preview was reading widgets that were eligible for
displaying in picker, which means widgets that are marked as hidden in
picker wouldn't show up in wallpaper preview.

This fix updates widgets model to maintain map of all widgets (instead of just pickable widgets like before), so that the existing `getWidgetsByComponentKey` function used by wallpaper preview can see all widgets. And, updates picker specific methods to use separate functions (suffixed `forPicker` that filter out picker ineligible widgets when read by picker code).

Bug: 385695615
Test: WidgetsModelTest, WidgetsPredictionUpdateTaskTest and demo
Flag: EXEMPT BUGFIX
Change-Id: I59efe38be0ce1f8a956ba4be42fb6e8b48b5d323