coregrilo: Properly reload the tracker plugin

This is a follow-up of commits
d7425fab and

All grilo plugins are loaded at startup. However, TrackerWrapper may
not be available yet. In that case, GrlTrackerWrapper is not
created. Once TrackerWrapper becomes available, all grilo plugins are
loaded again in order to instantiate GrlTrackerWrapper.
It means that the Tracker plugin is activated two times.

This issue was already solved by only reloading the tracker plugin when
TrackerWrapper becomes
available (commit 11f79699).
It calls the unload_plugin method from the registry and then
immediatly the activate_plugin_by_id method.
However, the unload_plugin call is asynchronous. So, the
activate_plugin_by_id call may not load reload the plugin because it
has not been deactivated yet.

The issue is fixed by calling the activate_plugin_by_id method once
the Tracker source is correctly removed.
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