Correctly receive files through Android DND

Correctly receive files through Android DND

* java/org/gnu/emacs/EmacsService.java (getUsefulContentResolver)
(getContentResolverContext): New functions which return a
content resolver from an EmacsActivity, if at all possible.
(openContentUri, checkContentUri): Probe or open URIs through
such content resolvers.  Probe URIs by opening them if merely
testing permissions fails, for DND URIs do not make
checkCallingUriPermission return true.

* java/org/gnu/emacs/EmacsWindow.java (onDragEvent): Address
potential crash.

* src/androidvfs.c (android_check_content_access): Circumvent
JNI dynamic method dispatch.
(android_authority_name): Guarantee NAME is never a directory.