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: I4431e5b0ed69b1027179d84a0f4327932bf3c599