Only keep user switcher bitmaps if needed

Only keeps icons if the user switcher is accessible. On phones, this
is any time quick settings is expanded. On tablets, this would be whenever
the keyguard shows. Reloading them on each screen off transition seems too
wasteful however, so we keep them in-memory all the time on tablets.

Bug: 18350670
Change-Id: I416beb82156787e61ee1f59b6726fd702f135f68