Mark implicit PendingIntents as immutable am: a5d52884d0 am: 55676b89da am: 6cae6e2d5f am: 933036b1d8

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/12296466

Change-Id: Idbd0e0a45b9305d1e05e3a8419d93e799064bd61