Avoid deadlock by disposing the accesibility context without the Mutex
locked, since it will eventually try to acquire the SolarMutex...

Thread 1 in UnoControl::getPosSize()
calling from sdr::contact::ControlHolder::getPosSize()

Thread 2 calling from UnoControl::setDesignMode()
trying to get SolarMutex in VCLXWindow::disposing()

Change-Id: I7d0ffe4fa0f8cd0c48e9b9b5e923ce229f97ca57