[user-mgmt]: Rearrange signal handler registration

Currently, signal handler are registered after the GetManagedObjects
query. This may miss any signals generated in-between this time. Correct
flow must be to register for the signal first, followed by
GetManagedObjects query.

Tested:
1. Verified that user list are properly listed after this change.
Note: Also, performed following tests to determine the order didn't cause any
problem. Verified with both ipmi_user.json deleted and preserved.
2. Started phosphor-user-manager after host-ipmid.
3. Re-started phosphor-user-manager.
4. Started phosphor-host-ipmid after phosphor-user-manager.
5. Restarted phosphor-host-ipmid.

Change-Id: I124b5e96672e0456289bca7a2b889e4b897c0545
Signed-off-by: arun-pm <arun.p.m@linux.intel.com>
2 files changed