Merge "Mark implicit PendingIntents as immutable" into rvc-dev am: e35ad478dc am: a64978fe65 am: 54d2facd6a

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/12296407

Change-Id: I7809dcb4d194009fb473bb01563c9a2fb3c3fb3c