Mark implicit PendingIntents as immutable am: 504e7d848d am: 46387fb17b am: 78c2d2ca94 am: b0e7b88525 am: c7bb9ba897

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/12296465

Change-Id: I07dc41ced50b174c72e4a6d6d3237b04ca959f5a