Only show collapse handle when the bottom widgets picker is scrollable

Test: Go to contact bottom widgets picker. No collapse handle is
      shown because the content isn't scrollable. Go to Google calendar
      bottom widget picker. Collapse handle is hown because the content
      is scrollable.
Bug: 185134663
Change-Id: Ic6109c055a072aa1cf3521f829833fd5f4cece0f

res/layout/widgets_bottom_sheet_content.xml

@@ -15,10 +15,12 @@
 -->
 <merge xmlns:android="http://schemas.android.com/apk/res/android">
     <View
+        android:id="@+id/collapse_handle"
         android:layout_width="48dp"
         android:layout_height="2dp"
         android:layout_gravity="center_horizontal"
         android:layout_marginBottom="16dp"
+        android:visibility="gone"
         android:background="?android:attr/textColorSecondary"/>
     <TextView
         style="@style/TextHeadline"

src/com/android/launcher3/widget/WidgetsBottomSheet.java

@@ -118,6 +118,7 @@ public class WidgetsBottomSheet extends BaseWidgetSheet implements Insettable {
             ViewGroup.LayoutParams layoutParams = widgetsTableScrollView.getLayoutParams();
             layoutParams.height = mMaxTableHeight;
             widgetsTableScrollView.setLayoutParams(layoutParams);
+            findViewById(R.id.collapse_handle).setVisibility(VISIBLE);
         }
     }