Mark implicit PendingIntents as immutable am: 7f0fd285c6 am: ae567e56d8 am: 347ec93c0e

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

Change-Id: I10dfcdb8b4f1a200c5b42b6dd817ee19f9569d40