Removed ActivityStack.

ActivityStack is not needed since two different types of Activities exist, Frame and Main. Each should be tracked by own wxWidgets class/Android class.
Passing  wxTopLevelWindow pointer to Activity.

In previous commit, by mistake i have added part of this code, wxAndroid::NewWindow is passed to Activity class. Here i have updated method argument's string.

Passing pointer is needed to allow Android to update corresponding wxWidgets class.