Add security-server.target unit.

Add security-server.target unit.

Service key-storage requires security-server but it should not use
security-server.socket in Requires field because it's part of private
implementation. This commit creates synchronization point
security-server.target that should be used by other services to add
requirement to security-server.

[Issue#]     N/A
[Bug/Cause]  N/A
[Problem]    key-storage is not able to start because new
             security-server does not create security-server.socket.
[Solution]   create synchronization point for key-storage

[Verification] Build, install, reboot device.

Change-Id: Id54e9187edf671eec047787bdeb27a8c2acca29e