Make some file I/O operations more robust on Android

Make some file I/O operations more robust on Android

* src/android.h (android_readlinkat): New prototype.
* src/androidvfs.c (struct android_vops): New function pointer
`readlink'.
(unix_vfs_ops, android_unix_readlink, afs_vfs_ops)
(android_afs_readlink, content_vfs_ops, android_content_mkdir)
(android_content_chmod, authority_vfs_ops)
(android_authority_readlink, saf_root_vfs_ops)
(android_saf_root_readlink, saf_tree_vfs_ops)
(android_saf_tree_readlink, saf_file_vfs_ops, saf_new_vfs_ops)
(android_saf_new_readlink, root_vfs_ops): Implement new
`readlink' vops for each vnode type.
(android_readlinkat): New function.

* src/fileio.c (Fcopy_file): Be more flexible about SELinux
errors under Android.
(emacs_readlinkat): Use `android_readlink' when built as
libemacs.so.