[automerger skipped] Revert "Revert "Set FLAG_IMMUTABLE flag on FirstScreenBroadcast PendingIntent. <snip> am: b1b76c34f5 -s ours

am skip reason: Merged-In I8d8e38a1aa6bdf13879d460cfa84cabe6c6bb1f2 with SHA-1 6a7a7f6bad is already in history

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

Change-Id: Ia0c3dee0bbab4383825e91fffd4b24acc0cb166d