Changes in TrackableAssets in experimental saved object graph.

 - Support exporting multiple TrackableAssets that refer to same filename.
 - Make SavedAsset proto depend on an external list of asset file defs that
 comes from a MetaGraphDef.

Note: This means that the MetaGraphDef supporting the object-based saved model view
must list all asset filedefs, even the ones that are not needed for exported signature
defs. As a consequence the exported meta graph must also have a feedable tensor
for each one of them, so old loaders do not fail when trying to digest the AssetFileDefs.
PiperOrigin-RevId: 224306775