Specify FLAG_IMMUTABLE when creating PendingIntent

Targeting S+ (version 31 and above) requires that one of
FLAG_IMMUTABLE or FLAG_MUTABLE be specified when creating a
PendingIntent. So this CL uses FLAG_IMMUTABLE to create
the PendingIntent because the flag is recommended in most cases.

Bug: 192103710
Test: manual
Change-Id: I4c9076e10ee6a6227ae9daea1e2f2ad3363671b8
diff --git a/src/com/android/car/systemupdater/UpdateLayoutFragment.java b/src/com/android/car/systemupdater/UpdateLayoutFragment.java
index ff56371..0ba5b90 100644
--- a/src/com/android/car/systemupdater/UpdateLayoutFragment.java
+++ b/src/com/android/car/systemupdater/UpdateLayoutFragment.java
@@ -279,7 +279,7 @@
                         context,
                         /* requestCode= */ 0,
                         intent,
-                        PendingIntent.FLAG_UPDATE_CURRENT);
+                        PendingIntent.FLAG_IMMUTABLE | PendingIntent.FLAG_UPDATE_CURRENT);
 
         return new Notification.Builder(context, NOTIFICATION_CHANNEL_ID)
                 .setVisibility(Notification.VISIBILITY_PUBLIC)