Make pendingintent use immutable flag

It will be required for S+ that every PendingIntent be declared
mutable or immutable.

Fixes: 170157567
Test: manual
Change-Id: I0abe399ec41371105a9613bebb82943958e918d7

packages/SystemUI/src/com/android/systemui/keyguard/KeyguardViewMediator.java

@@ -1007,7 +1007,7 @@ public class KeyguardViewMediator extends SystemUI implements Dumpable {
         intent.putExtra("seq", mDelayedShowingSequence);
         intent.addFlags(Intent.FLAG_RECEIVER_FOREGROUND);
         PendingIntent sender = PendingIntent.getBroadcast(mContext,
-                0, intent, PendingIntent.FLAG_CANCEL_CURRENT);
+                0, intent, PendingIntent.FLAG_CANCEL_CURRENT |  PendingIntent.FLAG_IMMUTABLE);
         mAlarmManager.setExactAndAllowWhileIdle(AlarmManager.ELAPSED_REALTIME_WAKEUP, when, sender);
         if (DEBUG) Log.d(TAG, "setting alarm to turn off keyguard, seq = "
                          + mDelayedShowingSequence);