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.

java/org/gnu/emacs/EmacsService.java

@@ -1769,4 +1769,29 @@ public final class EmacsService extends Service
 	    ? DocumentsContract.getDocumentId (name)
 	    : null);
   }
+
+  /* Return if there is a content provider by the name of AUTHORITY
+     supplying at least one tree URI Emacs retains persistent rights
+     to access.  */
+
+  public boolean
+  validAuthority (String authority)
+  {
+    List<UriPermission> permissions;
+    Uri uri;
+
+    permissions = resolver.getPersistedUriPermissions ();
+
+    for (UriPermission permission : permissions)
+      {
+	uri = permission.getUri ();
+
+	if (DocumentsContract.isTreeUri (uri)
+	    && permission.isReadPermission ()
+	    && uri.getAuthority ().equals (authority))
+	  return true;
+      }
+
+    return false;
+  }
 };