Adjust size and preview of the weather widget in platform samples.

- 4x2 is the recommended size for such widget
- avoid 3x? sizes
- Uses glance loading layout resource instead of the local xml
- Update the preview image

Author: shamalip

samples/user-interface/appwidgets/src/main/res/drawable-nodpi/weather_forecast_widget_preview.png

@@ -27,7 +27,7 @@
     android:previewImage="@drawable/weather_forecast_widget_preview"
     android:previewLayout="@layout/widget_weather_forecast_small"
     android:resizeMode="horizontal|vertical"
-    android:targetCellWidth="3"
+    android:targetCellWidth="4"
     android:targetCellHeight="2"
     android:updatePeriodMillis="86400000"
     android:widgetCategory="home_screen" />

samples/user-interface/appwidgets/src/main/res/layout/widget_loading.xml

@@ -15,15 +15,15 @@
   -->
 <appwidget-provider xmlns:android="http://schemas.android.com/apk/res/android"
     android:description="@string/app_widget_weather_description"
-    android:initialLayout="@layout/widget_loading"
+    android:initialLayout="@layout/glance_default_loading_layout"
     android:minWidth="184dp"
     android:minHeight="184dp"
     android:minResizeWidth="120dp"
     android:minResizeHeight="120dp"
     android:previewImage="@drawable/weather_forecast_widget_preview"
     android:previewLayout="@layout/widget_weather_forecast_small"
     android:resizeMode="horizontal|vertical"
-    android:targetCellWidth="3"
+    android:targetCellWidth="4"
     android:targetCellHeight="2"
     android:updatePeriodMillis="1800000"
     android:widgetCategory="home_screen" />