【FLAG_ACTIVITY_CLEAR_TASK | FLAG_ACTIVITY_NEW_TASK】
1、FLAG_ACTIVITY_NEW_TASK
2、FLAG_ACTIVITY_CLEAR_TASK
this flag will cause any existing task that would be associated with the activity to be cleared before the activity is started. That is, the activity becomes the new root of an otherwise empty task, and any old activities are finished.
这两个结合在一起，相当于清空一个backstack，将新activity置于root。
参考：http://android.xsoftlab.net/reference/android/content/Intent.html#FLAG_ACTIVITY_CLEAR_TASK
