Fix bug maclient info gets appended, not replaced on pkgmgr event

Fix bug maclient info gets appended, not replaced on pkgmgr event

When a packge manager event on dependent modules is detected
(such as wakeup engine package), the multi-assistant-service
tries to reload wakeup manager plugin and also reload the
assistant info files.
But the previous implementation appends assistant information
whenever this reload process is requested, not replacing the
original assistant information struct, causing the struct
to contain duplicated entries and also retains entries for
outdated assistant information.
For this reason, make sure the assistant information struct
is cleared before reading assistant info files.

Change-Id: I9ba1c4d7f4fd8c1070f9e1dd5d9d63fc002dd380