Fixed the rendering delays bug when app is first opened

[nketia]

There was about 5 seconds delay rendering the app-intro-slider when an app is first launched when tested on iOS. This delay wasn't happening in the earlier version of 2.0.1. This pull request is to fix that initial delay.

[hkar19]

is it better to have width and height initially in state rather than on onLayout?

[nketia]

is it better to have width and height initially in state rather than on onLayout?

Yes. Having the width and height set in the state to window figures solves the issue.

[hkar19]

i think due to this issue #224 , it is better to set width and height early to Dimensions.get('screen') values and keep setting width and height to state on onLayout,

as it will potentially break the goToSlide function. concerning line.

[hkar19]

#237