Mark implicit PendingIntents as immutable am: 504e7d848d am: 46387fb17b am: 78c2d2ca94 am: b0e7b88525 am: c7bb9ba897 am: e5d90630bd am: 4d38a90bf2

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

Change-Id: I7a70cc908aa94ce10e615a592ff2077e63b6c056