Mark implicit PendingIntents as immutable am: a5d52884 am: 55676b89 am:...

Mark implicit PendingIntents as immutable am: a5d52884 am: 55676b89 am: 6cae6e2d am: 933036b1 am: 2aaf9b4b am: 6f545b91 am: 4f4f5fb6 am: 527435e2

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

Change-Id: Iae96a677844f4b1ec634d1152ba1d0c4bb23d9e4