Android: Show a message when adding a folder with no games

To catch people who try to use unsupported formats.

Source/Android/app/src/main/res/values/strings.xml

@@ -438,6 +438,7 @@ It can efficiently compress both junk data and encrypted Wii data.
     <string name="no_file_extension">The selected file does not appear to have a file name extension.\n\nContinue anyway?</string>
     <string name="wrong_file_extension_single">The selected file has the file name extension \"%1$s\", but \"%2$s\" was expected.\n\nContinue anyway?</string>
     <string name="wrong_file_extension_multiple">The selected file has the file name extension \"%1$s\", but one of these extensions was expected: %2$s\n\nContinue anyway?</string>
+    <string name="wrong_file_extension_in_directory">No compatible files were found in the selected location.\n\nThe supported formats are: %1$s</string>
     <string name="unavailable_paths">Dolphin does not have permission to access one or more configured paths. Would you like to fix this before starting?</string>
 
     <!-- Misc -->