Register package manager event handler before loading wakeup engines

Register package manager event handler before loading wakeup engines

If the wakeup engine app gets installed while
multi-assistant-service is being launched and also
is trying to load wakeup engines, the newly installed
wakeup engine will not be loaded since at the time of
launching the installation was not completed,
and the installation completion event would not be
received also, since the package manager event handler
might not be registered yet.
So if we register package manager event handler first
and then load wakeup engines, even if the wakeup engine
currently being installed fails to be loaded, the
installation completion event is guaranteed to be received
by the package manager event handler, making sure that
the multi-assistant-service would restart for re-loading.

Change-Id: I1e8f57d6a08c9290a0ae11422d1404188c97c3df