Initialize DataManager on boot complete.

Perfetto traces were showing PeopleService.onUnlockUser was often
blocked on monitor contention for the ShortcutService
shortcutHandleUnlockUser thread to finish.

Move DataManager initialization from onUnlockUser to
PHASE_BOOT_COMPLETE so it can add its ShortcutServiceCallback before
shortcutHandleUnlockUser starts. That takes the relatively slow
shortcutHandleUnlockUser off the critical path for boot.

Bug: 389194339
Test: Boot device, confirm monitor contention is gone via Perfetto trace.
Test: atest com.android.server.people.PeopleServiceTest
Flag: com.android.server.flags.early_data_manager_init

Change-Id: I89d1259ad14d33b53561fb97082b054c11a0d659