android.uid.system不能访问sdcard

转自：http://www.eoeandroid.com/forum.php?mod=viewthread&tid=63314&page=1#pid636737

http://groups.google.com/group/android-porting/browse_thread/thread/c6af65d7ddd5eff3

http://stackoverflow.com/questions/5617797/android-shared-user-id-and-reading-writing-a-file

http://stackoverflow.com/questions/6679011/it-cant-access-the-sdcard-after-setting-android-uid-system-in-my-application

Processes that continue holding open fds on the sdcard a little after it is
requested to be unmounted will be killed so that it can unmount.

We don't want the system process to  be able to access the sdcard to avoid
these kinds of issues (and just general security cleanliness), so that it
does not have permission to access it.