Add @Nullable annotation to AllAppsStore.getApp. am: e1a8440d78

Original change: https://googleplex-android-review.googlesource.com/c/platform/packages/apps/Launcher3/+/13563273

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Ia6a826ef54996f5b105b40bafd8e97351dbb33d8