Merge "Mark implicit PendingIntents as immutable am: 7f0fd285 am: ae567e56 am: cff7b95c am: 325c62d8" into rvc-qpr-dev-plus-aosp am: eb946dd4

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/12296406

Change-Id: Ibb2bb12ec99c280e583352376291dc51df95f9e7