Default to using SplitSelectSource drawable if TaskView icon drawable is null

* Alternative solution could be to set
onTaskViewVisibilityChanged(true) for the taskView that is about
to be dismissed so it loads it's taskIcon/thumbnail back from the cache
* However, that does still leave us open to race conditions (even though
we can be reasonably confident the icon is probably in the cache)
* Also made other changes to allow already public fields on some classes
to be mockable for unit testing

Fixes: 275267738
Test: Tested with fullscreen task at end of overview,
GroupedTaskView at end of overview,
Initiating split from home,
Initiating split from overview actions,
Initiating split from overview app icon

Change-Id: Ic9059c93c07b90f61c9f418d5d36d6ba201ff96a

src/com/android/launcher3/util/SplitConfigurationOptions.java

@@ -200,8 +200,8 @@ public final class SplitConfigurationOptions {
         /** Keep in sync w/ ActivityTaskManager#INVALID_TASK_ID (unreference-able) */
         private static final int INVALID_TASK_ID = -1;
 
-        public final View view;
-        public final Drawable drawable;
+        private View view;
+        private Drawable drawable;
         public final Intent intent;
         public final SplitPositionOption position;
         public final ItemInfo itemInfo;
@@ -224,5 +224,13 @@ public final class SplitConfigurationOptions {
             this.itemInfo = itemInfo;
             this.splitEvent = splitEvent;
         }
+
+        public Drawable getDrawable() {
+            return drawable;
+        }
+
+        public View getView() {
+            return view;
+        }
     }
 }