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

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

Change-Id: Ia9ffe0505707ac66cb2cd8864bf77bc6c361fdc2