Add null check to Task#equals()

Adds a couple base conditions to Task#equals -- essentially so that if the comparison target is null, returns false.

In Java Platform SE 8 docs, Object#equals() implements "For any non-null reference value x, x.equals(null) should return false. Since Task#equals() overrides Object.equals(), we should implement this too.

Bug: 274835596
Test: Manual
Change-Id: If8e23f620c4f2c88d1e058325fdebc54022acf3b

packages/SystemUI/shared/src/com/android/systemui/shared/recents/model/Task.java

@@ -336,6 +336,14 @@ public class Task {
 
     @Override
     public boolean equals(Object o) {
+        if (o == this) {
+            return true;
+        }
+
+        if (!(o instanceof Task)) {
+            return false;
+        }
+
         // Check that the id matches
         Task t = (Task) o;
         return key.equals(t.key);