Composite background color is not handled consistently between android / ios / browser

- On Android a tree inherits the background color of the parent
- On iOs it is initialized to the default color.

Expected: the code snippet below should have the same result on both platforms.

``` java
package app;

import org.eclipse.swt.SWT;
import org.eclipse.swt.layout.FillLayout;
import org.eclipse.swt.widgets.Composite;
import org.eclipse.swt.widgets.Text;
import org.eclipse.swt.widgets.Tree;

import com.eclipsesource.tabris.ui.AbstractPage;
import com.eclipsesource.tabris.ui.PageData;

@SuppressWarnings("serial")
public class TopLevelPage extends AbstractPage {

private Text text;

@Override
public void createContent(final Composite parent, PageData data) {
parent.setBackground(parent.getDisplay().getSystemColor(SWT.COLOR_RED));
parent.setLayout(new FillLayout());

Composite c = new Composite(parent, SWT.NONE); // also for Tree
}

}
```