Mark implicit PendingIntents as immutable am: 7f0fd285c6 am: ae567e56d8 am: cff7b95c37 am: d135ad5fef

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

Change-Id: I39c4747aa46b827466d3456a5e9b06077bf72f67