Query the number of presentation displays when extension is created.

In some case, e.g. in cordova container, onResume method won't get
called at startup, it is called only when resuming from background
status.

This fix can make sure that presentation.displayavailable flag won't
depend on the fact if PresentationExtension::onResume gets executed.
Merge pull request #1310 from hmin/hot-fix-presentation-api

Query the number of presentation displays when extension is created.