Mark implicit PendingIntents as immutable am: a5d52884d0 am: 55676b89da am: 6cae6e2d5f am: 933036b1d8 am: 2aaf9b4b77 am: 6f545b916f am: 4f4f5fb68f

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

Change-Id: I07f45b2091352f21f28ddcea129806b4216cb4ac