Mark implicit PendingIntents as immutable am: 599c3e4b75 am: fa64ffd0a2 am: 3ff2a5cbd1 am: ec741cf6d2 am: 9058493863

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

Change-Id: Id5566fbfc4f5993d36e672d1ccf6037e7b945b79