Mark implicit PendingIntents as immutable am: 7f0fd285c6 am: ae567e56d8 am: cff7b95c37 am: 325c62d876

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

Change-Id: I045448a8f014536ce39fe51f36e4e1ab5bfcc74a