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: I97946675a0cad34c0e6852ac4c28e8a7f8096c5a