Mark implicit PendingIntents as immutable am: 7f0fd285c6 am: ae567e56d8 am: 347ec93c0e am: 438c8773b0 am: a177dd002b

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

Change-Id: I6c67d6cdabc8ca844057f3608602936c0d0fe061