Fix BackupManager#getUserForSerialNumber

Test: atest ProfileSerialNumberHostSideTest

Currently BackupManager#getUserForSerialNumber goes through users
associated with the calling user (inluding the calling user itself) and
looks for the given serial number. However, to get the list of users it calls
UserManager#getUserProfiles that operates for the users associated with
the calling process (in our case - system user since the call is made
from BMS). While this works for our current use case (only support
backup for system user and their work profile, meaning
BackupManager#getUserForSerialNumber will only be called for system
user), it won't work for a full multi-user case.

Change-Id: Ia42a69a1216ed9a5dae596e44336290d2718de7a