通过延长开机动画，等进入到非DirectBoot模式，再结束开机动画。

frameworks\base\services\core\java\com\android\server\wm\ActivityRecord.java

  void onWindowsDrawn(boolean drawn, long timestampNs) {
        mDrawn = drawn;
        if (!drawn) {
            return;
        }

        //Launcher is drawn completed,box can exit bootanim
        if ("box".equals(SystemProperties.get("ro.target.product"))){
            if(shortComponentName!=null && !shortComponentName.contains(".FallbackHome")
                && !"1".equals(SystemProperties.get("service.bootanim.exit"))){
                    stopBootanim();
            }
        }

        final TransitionInfoSnapshot info = mStackSupervisor
                .getActivityMetricsLogger().notifyWindowsDrawn(this, timestampNs);
        final boolean validInfo = info != null;
        final int windowsDrawnDelayMs = validInfo