Mark implicit PendingIntents as immutable am: db245b0310 am: 449ad4e208 am: 8add5d8b8f am: 846f484b98

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/12296404

Change-Id: I38300642a9d58fb29b1eba2f67a8583db37d760e