Mark implicit PendingIntents as immutable am: 504e7d848d am: 46387fb17b am: 78c2d2ca94

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/12296465

Change-Id: Ic3d883d4effd8e01c950c8554aba670894a5e634