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: Ied126ae6a371fdd27074557500e328a7b1564fe5