Mark implicit PendingIntents as immutable am: db245b0310 am: 449ad4e208 am: 8add5d8b8f am: 846f484b98 am: c8eee458d0

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

Change-Id: I3c080d8a6220c9256a2d4e4940628a3dea3ea6b0