Add Nullable annotation import to fix the build am: a399403e am: 59b14846

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/18756206



Change-Id: Id4165b031ee1560eec281a1d784f5a1a4617d851
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>