Mark implicit PendingIntents as immutable am: db245b0310 am: 449ad4e208 am: 8add5d8b8f am: 846f484b98 am: c8eee458d0 am: c8d58526dd

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

Change-Id: Ia355ecba05496577ea56408d3cf67b3e917001b9