Add title to exported GPX metadata

When the Caltopo app exports a GPX file, the title of the map is lost, because "Share" on Android does not preserve the filename (it passes a content:// intent with a meaningless name).  Instead, Caltopo should put the title of the map in the <name> tag in the <metadata> section of the GPX file, where other apps can parse it.

Here's the spec of the <metadata> section:

https://www.topografix.com/GPX/1/1/#type_metadataType