扩展Mobile.back()方法，支持子应用退回主应用的场景

wade-mobile-func/src/com/wade/mobile/func/MobileUI.java

@@ -411,6 +411,13 @@ public class MobileUI extends Plugin {
		if (mainFlipper != null) {
//			Runtime runtime = Runtime.getRuntime();
//			runtime.exec("input keyevent " + KeyEvent.KEYCODE_BACK);
		    
            // add by chengwb3 android子应用回退主应用使用
            if (!mainFlipper.isCanBack()) {
                context.finish();
                return;
            }
            
			final String tag = param.getString(0);
			if(!isNull(tag)){
				context.runOnUiThread(new Runnable() { public void run() {