Fix visiting and saving writable content provider files

Fix visiting and saving writable content provider files

* java/org/gnu/emacs/EmacsService.java (checkContentUri):
Improve debug output.
* lisp/files.el (basic-save-buffer): Check whether or not file
itself exists before checking for the existence of the directory
containing it.
* src/android.c (android_open): Don't forget to set errno after
open_content_uri fails.