Isolate fchmodat within the Android VFS layer

Isolate fchmodat within the Android VFS layer

* src/android.h: Update prototypes.
* src/androidvfs.c (unix_vfs_ops, android_unix_chmod, afs_vfs_ops)
(android_afs_chmod, content_vfs_ops, android_content_chmod)
(authority_vfs_ops, android_authority_chmod, saf_root_vfs_ops)
(android_saf_root_chmod, saf_tree_vfs_ops, android_saf_tree_chmod)
(saf_file_vfs_ops, saf_new_vfs_ops, android_saf_new_chmod)
(root_vfs_ops): Add `chmod' to the list of functions implemented
by each vnode.
(android_fchmodat): New function.
* src/fileio.c (Fset_file_modes): Use `emacs_fchmodat'.
* src/lisp.h:
* src/sysdep.c (emacs_fchmodat): Delegate to android_fchmodat on
Android.