Add activity option flag for background starts for bubbles

Seems like something changed in udc that means we need this now.

A bubble only gets opened due to a user interaction so it should
be fine to use this flag.

Bug: 270058419
Test: manual - get an SMS bubble, tap on it, observe content loads
Change-Id: I6f8eaab793dc2fde45e097bbc1de38c995be02d6

libs/WindowManager/Shell/src/com/android/wm/shell/bubbles/BubbleExpandedView.java

@@ -16,6 +16,7 @@
 
 package com.android.wm.shell.bubbles;
 
+import static android.app.ActivityOptions.MODE_BACKGROUND_ACTIVITY_START_ALLOWED;
 import static android.app.ActivityTaskManager.INVALID_TASK_ID;
 import static android.content.Intent.FLAG_ACTIVITY_MULTIPLE_TASK;
 import static android.content.Intent.FLAG_ACTIVITY_NEW_DOCUMENT;
@@ -226,6 +227,8 @@ public class BubbleExpandedView extends LinearLayout {
                 try {
                     options.setTaskAlwaysOnTop(true);
                     options.setLaunchedFromBubble(true);
+                    options.setPendingIntentBackgroundActivityStartMode(
+                            MODE_BACKGROUND_ACTIVITY_START_ALLOWED);
 
                     Intent fillInIntent = new Intent();
                     // Apply flags to make behaviour match documentLaunchMode=always.