2D Recents: Show shadows behind task views.

Test: Checked that there's a shadow behind each task view in Grid-based
Recents on local sw600dp device.
Bug: 32101881
Change-Id: I2cb2c0f73cbe23c4dcce5a4839965fdff13a0df5

packages/SystemUI/src/com/android/systemui/recents/views/grid/TaskGridLayoutAlgorithm.java

@@ -145,8 +145,11 @@ public class TaskGridLayoutAlgorithm  {
         int y = gridInfo.yOffsets[taskIndex];
         float z = stackLayout.mMaxTranslationZ;
 
+        // We always set the dim alpha to 0, since we don't want grid task views to dim.
         float dimAlpha = 0f;
-        float viewOutlineAlpha = 0f;
+        // We always set the alpha of the view outline to 1, to make sure the shadow is visible.
+        float viewOutlineAlpha = 1f;
+
         // We also need to invert the index in order to display the most recent tasks first.
         int taskLayoutIndex = taskCount - taskIndex - 1;
         boolean isTaskViewVisible = (taskLayoutIndex < MAX_LAYOUT_TASK_COUNT);