Updates to not include the status bar in the root view for Android.

Previously the Window::GetRootView() method returns the view including status bar. This commit changes to return its first child view that doesn't include status bar.

This commit also moves the implementation of the Window class from C++ to Java which would improve the readability of Java integration.

Some comments in the platform.cc file are also fixed as well.

Signed-off-by: Olli Wang <olliwang@ollix.com>