[Android] Remove singleton architecture

This pull request targets the Issue #17 
the initialization of a static member has to have an explicit static initiallization block.
```
private static SparseArray<HybridgeBroadcaster> sClients;
static {
    //static initialization 
    sClients = new SparseArray<HybridgeBroadcaster>();
}
```

if not, each time the current activity context is removed, the static member will be removed