Mark implicit PendingIntents as immutable am: db245b0310 am: 449ad4e208 am: 8add5d8b8f am: 846f484b98 am: c8eee458d0 am: 5b5b6fb273 am: 3e941684da

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/12296404

Change-Id: Ieee176d91e94dc768040b51b77ca2e0efd84ac8e