public static final int MODE_PRIVATE
Added in API level 1
File creation mode: the default mode, where the created file can only be accessed by the calling application (or all applications sharing the same user ID).
public static final int MODE_APPEND
Added in API level 1
File creation mode: for use with openFileOutput(String, int), if the file already exists then write data to the end of the existing file instead of erasing it.
从SDK对于这两个常量的描述来看，他们控制的功能不一样
MODE_APPEND 的功能是判断是否有该文件，如果有在后面添加，而不是擦除
MODE_PRIVATE 该文件只能被创建他的应用访问（控制访问权限）
功能不同