bug描述：如果当前app是全屏的属性，导致状态栏隐藏且有dialog 弹出时，
这个情况下想下拉显示状态栏，会导致状态栏频繁闪烁。
//services/core/java/com/android/server/wm/DisplayPolicy.java
//更新系统状态栏的属性
int updateSystemUiVisibilityLw() {
// If there is no window focused, there will be nobody to handle the events
// anyway, so just hang on in whatever state we're in until things settle down.
//只有焦点窗口才能控制系统栏，没有焦点的窗口，不会响应这些事件
WindowState winCandidate = (mFocusedWindow != null) ? mFocusedWindow : mTopFullscreenOpaqueWindowState;
//所以可以在这里打印日志，看出现bug时焦点窗口的情况，然后分析
android.util.Log.d("TAG",(mFocusedWindow != null)+"");
android.util.Log.d("TAG",mFocusedWindow.toString()+"");//打印当前获取焦点的窗口信息：app的包名
if (winCandidate == null) {
return 0;
}
//add
if (mFocusedWindow != null) {
if (mFocusedWindow.getAttrs().token == null) {
winCandidate = mTopFullscreenOpaqueWindowState;
}
}
//end
// The immersive mode confirmation should never affect the system bar visibility, otherwise
// it will unhide the navigation bar and hide itself.
if (winCandidate.getAttrs().token == mImmersiveModeConfirmation.getWindowToken()) {
...........
}
................
}