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.

java/org/gnu/emacs/EmacsWindow.java

@@ -1601,7 +1601,7 @@ public final class EmacsWindow extends EmacsHandleObject
   {
     ClipData data;
     ClipDescription description;
-    int i, x, y;
+    int i, j, x, y, itemCount;
     String type;
     Uri uri;
     EmacsActivity activity;
@@ -1626,11 +1626,12 @@ public final class EmacsWindow extends EmacsHandleObject
 
 	data = event.getClipData ();
 	description = data.getDescription ();
+	itemCount = data.getItemCount ();
 
 	/* If there are insufficient items within the clip data,
 	   return false.  */
 
-	if (data.getItemCount () < 1)
+	if (itemCount < 1)
 	  return false;
 
 	/* Search for plain text data within the clipboard.  */
@@ -1662,12 +1663,14 @@ public final class EmacsWindow extends EmacsHandleObject
 	      {
 		/* If the item dropped is a URI, send it to the main
 		   thread.  */
+
 		uri = data.getItemAt (0).getUri ();
 
 		/* Attempt to acquire permissions for this URI;
 		   failing which, insert it as text instead.  */
 
-		if (uri.getScheme () != null
+		if (uri != null
+		    && uri.getScheme () != null
 		    && uri.getScheme ().equals ("content")
 		    && (activity = EmacsActivity.lastFocusedActivity) != null)
 		  {