When starting up abilities, there is a race condition between OnSurfaceChanged
and RunPage, which both are executed as PostTask on the same task executor.
This executor works in FIFO mode, and if (when) the OnSurfaceChanged comes
first, it will fail with a nullptr check on runningPage_.  This is because the
runningPage_ variable is set by the RunPage PostTask.

This patch is obviously just a workaround, and not a proper fix.  Hopefully, 500
ms will be enough in all relevant situations, but at least in theory, the
problem can still appear.

Signed-off-by: Esben Haabendal <esben@geanix.com>