[automerger skipped] Set FLAG_IMMUTABLE flag on FirstScreenBroadcast PendingIntent. am: 5af0edefdb am: 5bd2f65258 -s ours am: f2947b8a06 -s ours

am skip reason: Merged-In I881a2b79462e8d420b3dcb6a6b98adbe9e8e0394 with SHA-1 afa327d176 is already in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/Launcher3/+/14936971

Change-Id: I9d1fd7a0797157b8ab6de3d6f2f3257311224b09