Mark implicit PendingIntents as immutable am: 7f0fd285c6 am: ae567e56d8 am: cff7b95c37

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/12296406

Change-Id: I3be622ac290e94d9b6e4fb609e34fbfa2bb041d3