获取activity顶层view,用于动态添加View(addView)

/**
 * 获取通过setContentView设置进去的view
 */
private fun getContentView(activity: Activity) = getRootView(activity).getChildAt(0) as ViewGroup

/**
 * 获取activity顶层View
 */
private fun getRootView(activity: Activity) = activity.findViewById(android.R.id.content)