Mark implicit PendingIntents as immutable am: 7f0fd285c6 am: ae567e56d8 am: 347ec93c0e am: 438c8773b0

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

Change-Id: I91161a8e6ea05e00f44e871792a4830dc217b6e5