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: I2bb91d98b443af2b5c16944ce197d863dbd33f9a

samples/SipDemo/src/com/example/android/sip/WalkieTalkieActivity.java

@@ -137,7 +137,7 @@ public class WalkieTalkieActivity extends Activity implements View.OnTouchListen
 
             Intent i = new Intent();
             i.setAction("android.SipDemo.INCOMING_CALL");
-            PendingIntent pi = PendingIntent.getBroadcast(this, 0, i, Intent.FILL_IN_DATA);
+            PendingIntent pi = PendingIntent.getBroadcast(this, 0, i, Intent.FILL_IN_DATA | PendingIntent.FLAG_IMMUTABLE);
             manager.open(me, pi, null);