[Android] Remove singleton architecture

This pull request targets the Issue #17 
It's confusing to have a "singleton instance creator". 
This is more a "HybridgeBroadcasterFactory", isn't it? In that case, this would be part of another class... And this would drive into "2 flavours" of using the Hybridge
1) Directly creating and destroying the HB, and
2) Using a Factory, that will manage the different instances in order to reuse the hybridge for each WebView 
This second approach requires a strict lifecycle management from the final application, since storing references to WebViews could drive to memory leaks (not removing a reference to a webview by not destroying the hybridge that references it...)