Add OfficeIPCThread::IsEnabled.

This lets us easily check whether Desktop::Main has shut down,
i.e. which happens on the first restart-requiring initialisation.

Change-Id: I7fe5699a4bf59e2c3b891ac73abd9b147773d22e
Fail LOK initialisation if restart is required.

Change-Id: Ia13a3523666147f002b5a5177fd6962d55556f56