Mark implicit PendingIntents as immutable am: 599c3e4b75 am: fa64ffd0a2 am: 3ff2a5cbd1

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/12296405

Change-Id: I0836135d575492f638248350e7811aad05a618bf