The ImplHandleGetObject() function modifies the Application Settings and
must take the SolarMutex for this; otherwise we get crashes when another
thread calls StyleSettings::GetPersonaHeader() during some unoapi test.

Also fix another HWND to long cast; use LRESULT which is Win32 for intptr_t.

Change-Id: I5580f88ed0a8e9955697655ace94584e2307982d