顺便处理了软键盘弹出收起时不能再次隐藏navigation和statusbar的问题
直接上代码：

private LinearLayout llMain;
    private int screenHeight;
    private WindowManager wm;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        wm = this.getWindowManager();

        //activity最外层layout
        llMain = (LinearLayout) findViewById(R.id.llMain);

        //这里监听键盘弹出收起状态
        llMain
                .getViewTreeObserver()
                .addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener(){
                    @Override
                    public void onGlobalLayout() {
                        Rect r = new Rect();
                        llMain.getWindowVisibleDisplayFrame(r);
                        if (r.bottom == screenHeight) {
                            fullScreenImmersive();
                        }
                    }
                });
    }

    //首次进入时隐藏navigation和statusbar
    @Override
    public void onWindowFocusChanged(boolean hasFocus) {
        super.onWindowFocusChanged(hasFocus);
        if (screenHeight == 0) {
            screenHeight = wm.getDefaultDisplay().getHeight();
        }
        if (hasFocus && Build.VERSION.SDK_INT >= 19) {
            fullScreenImmersive();
        }
    }

    //沉浸式关键代码
    private void fullScreenImmersive() {
        if (Build.VERSION.SDK_INT >= 19) {
            View decorView = getWindow().getDecorView();
            decorView.setSystemUiVisibility(
                    View.SYSTEM_UI_FLAG_LAYOUT_STABLE
                            | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
                            | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                            | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
                            | View.SYSTEM_UI_FLAG_FULLSCREEN
                            | View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY);
        }
    }