unlock should always return TargetLock

unlock should always return TargetLock

ControlLoopEventManager.unlock only returns the TargetLock if
the PolicyGuard.unlock operation is successful.  However, if
the lock has already expired, then PolicyGuard.unlock will return
false, and ControlLoopEventManager.unlock will then return null.
As a result, the rules do not retract the TargetLock.  (The rules
do, however, retract any left-over TargetLock objects during the
clean-up phase.  Nevertheless, the earlier rules should not have
to depend on the clean-up rules to do that.)
Modified the code to address this issue.

Change-Id: I2e6099a4e3511053d2e6e1373cae4b480798d1b6
Issue-ID: POLICY-872
Signed-off-by: Jim Hahn <jrh3@att.com>