Add Nullable annotation import to fix the build am: a399403e

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



Change-Id: I12892de098717e0fbd5301c9410d53255170125d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>