Update Android port

Update Android port

* doc/emacs/android.texi (Android File System): Describe how to
access real files named /assets or /contents if so required.
* java/org/gnu/emacs/EmacsService.java (validAuthority):
* src/android.c (android_init_emacs_service):
* src/android.h: New function.
* src/androidvfs.c (android_saf_valid_authority_p): New
function.  Wrap the Java function.
(android_saf_root_stat, android_saf_root_access): Don't return
success if no authority by vp->authority's name exists.
(android_saf_tree_from_name): Check validity of string data
before giving it to JNI.