[iOS] Fix statusbar color while in a conference

Be explciit about the appearance we desire, since each mounted StaturBar
component will override the existing values. In this case, the problem was
caused because the default on iOS is dark, whereas it's light on Android.

Set it to light so it works consistently across both, which is what we want.

react/features/conference/components/Conference.native.js

                 accessible = { false }
                 style = { styles.conference }>
                 <StatusBar
+                    barStyle = 'light-content'
                     hidden = { true }
                     translucent = { true } />