Mark implicit PendingIntents as immutable am: 599c3e4b75 am: fa64ffd0a2 am: 3ff2a5cbd1 am: 6cb8e4a157 am: 31d1d2162e

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

Change-Id: I836177e7b19cacd781b0a7fd190dffbf537e5bf9