Add hashCode method to BrightnessReason

It implements equals, so it almost certainly should implement hashCode.

Bug: 217923092
Test: manual
Change-Id: I6acaea66d97231bd515df8128a7abd4aa067d599

services/core/java/com/android/server/display/DisplayPowerController.java

@@ -75,6 +75,7 @@ import com.android.server.display.whitebalance.DisplayWhiteBalanceSettings;
 import com.android.server.policy.WindowManagerPolicy;
 
 import java.io.PrintWriter;
+import java.util.Objects;
 
 /**
  * Controls the power state of the display.
@@ -2824,13 +2825,18 @@ final class DisplayPowerController implements AutomaticBrightnessController.Call
 
         @Override
         public boolean equals(Object obj) {
-            if (obj == null || !(obj instanceof BrightnessReason)) {
+            if (!(obj instanceof BrightnessReason)) {
                 return false;
             }
             BrightnessReason other = (BrightnessReason) obj;
             return other.reason == reason && other.modifier == modifier;
         }
 
+        @Override
+        public int hashCode() {
+            return Objects.hash(reason, modifier);
+        }
+
         @Override
         public String toString() {
             return toString(0);