Mark implicit PendingIntents as immutable am: a5d52884d0 am: 55676b89da am: 6cae6e2d5f

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/12296466

Change-Id: Ifbc28a64dab7015355b57d41514d1e0b670c5db1