Added MainActivity java file required for running wxWidgets.

MainActivity is used by wxApp and controls application lifecycle from Android's side. It contains small comments explaining how to include and run application using wxWidgets.
Added FrameActivity class required by wxFrame and wxDialog.

FrameActivity connects to wxTopLevelWindow(wxFrame and wxDialog to be precise). MainActivity opens this activity each time new wxFrame/wxDialog(wxTopLevelWindow) calls Show().