Add FLAG_IMMUTABLE to PendingIntents

If your app targets Android 12, you must specify the mutability of each
PendingIntent object that your app creates. This additional requirement
improves your app's security.
https://developer.android.com/about/versions/12/behavior-changes-12#pending-intent-mutability

Android Lint is being updated, and adds an error-level check for this.

Bug: 247885568
Test: Presubmits
Change-Id: Ib82e2b0bbea3c1dd1810c56c4ea70f7809aa122a

tests/FixVibrateSetting/src/com/android/fixvibratesetting/FixVibrateSetting.java

@@ -110,7 +110,7 @@ public class FixVibrateSetting extends Activity implements View.OnClickListener
 
     private void test() {
         Intent intent = new Intent(this, FixVibrateSetting.class);
-        PendingIntent pending = PendingIntent.getActivity(this, 0, intent, 0);
+        PendingIntent pending = PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_IMMUTABLE);
 
         Notification n = new Notification.Builder(this)
                 .setSmallIcon(R.drawable.stat_sys_warning)