Avoid calling potentially pure virtual method

Avoid calling potentially pure virtual method

In Qt 5.4 screenChanged is called indirectly from the destructor of
QPlatformScreen. By comparing new values against the oldScreen we call
call virtual methods of QPlatformScreen from it's own destructor which
results in a crash.

This patch simply emits change signals whenever a screen change regardless
of whether the value differs from the previous screen. Arguably less
efficient, but better than crashing.

This fix is not needed in Qt 5.5 where the QPA architecture has changed.

Task-number: QTBUG-45753
Change-Id: Ic155906928855a377add9b21bff9e72b31f4667e
Reviewed-by: Alan Alpert <aalpert@blackberry.com>