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

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

Change-Id: I836249b9be43c6d7ce46083ce88979fb9418c495