Minor adjustments to Android drag and drop and content URIs

Minor adjustments to Android drag and drop and content URIs

* java/org/gnu/emacs/EmacsWindow.java (EmacsWindow)
<dndXPosition, dndYPosition>: New fields initialized to -1.
(onDragEvent): Remember the position of the previous event to
avoid sending duplicates.

* src/androidvfs.c (EMACS_PATH_MAX): New define.
(android_saf_tree_rename, android_saf_tree_opendir)
(android_name_file, android_fstatat, android_faccessat)
(android_fchmodat, android_readlinkat): Use EMACS_PATH_MAX where
SAF file names might be encountered.