Enable user of UserWakeupStore only on devices that support multiuser.

Since devices that do not support multiuser do not have secondary users, there is no need to store wakeups on them.

Bug: 352696527
Test: atest AlarmManagerServiceTest
Flag: com.android.server.alarm.start_user_before_scheduled_alarms
Change-Id: Id1b7d4ae514de42c21e1fdee670d36a88dbeae3d