Fix NPE when loading recently opened apps on the Apps page

From Android U, Settings start to support showing work profile apps on
recently opened apps list on the Apps page. It will query the app entry
from the cache of ApplicationsState by different user id before cache
initialization. We should check mEntriesMap by user id in getEntry() and
getEntryLocked() to avoid NPE since the cache might be empty before
reading.

Bug: 284252546
Test: robotests
Change-Id: Ie4dc0821c4fe29a389f491eb4706abe352c05c5a