[automerger skipped] Revert "Revert "Set FLAG_IMMUTABLE flag on FirstScreenBroadcast PendingIntent. <snip> am: b1b76c34 -s ours am: 55e7fc68 -s ours

am skip reason: Merged-In I8d8e38a1aa6bdf13879d460cfa84cabe6c6bb1f2 with SHA-1 6a7a7f6b is already in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/Launcher3/+/14936970

Change-Id: Ia4831447289e414247ddf490e0beb505f3b72e08