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Don't use faccessat(AT_SYMLINK_NOFOLLOW). In Android, AT_SYMLINK_NOFOLLOW
is ignored. In glibc, it returns counter intuitive results when a
symbolic link is encountered, returning true all the time even though
an open(O_NOFOLLOW) will eventually fail.

Instead, stat the file and check to see if it's a regular file,
not a directory or symlink or some other weirdness.

In addition, fix a bug where isAsecInDirectory would return
true ("-1") if the asec directory didn't exist. It should return
false.

Bug: 18867827
Change-Id: I33d90e9095fad36ce0f83fde105b70f72e4eaef4