[Documentation] Not clear how to run demo apps

[dturner]

I was trying to run the demo-cast app. I imported the demos/demo-cast folder into Android Studio and did a gradle sync which produced the following error:

Task 'wrapper' not found in project 'demos-cast'

I realised that in order to run the demos one must import the ExoPlayer root project into Android Studio, rather than the demo subfolder.

It'd be good if this was made clear in the demo README as it would save some time and headscratching when attempting to run a demo app.