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

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

Change-Id: I22fca35b35ce74d28f0777206a383f7c7927a295