Mark implicit PendingIntents as immutable am: 599c3e4b75 am: fa64ffd0a2 am: 3ff2a5cbd1 am: 6cb8e4a157 am: 1af73a8535

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

Change-Id: I0e2265dfe2af0784c6c478ed3bb669a996e8d375