Backup and restore options for core_xapi

In MDL-67789, the xAPI State support has been implemented. 

During the course backup and restore process, xAPI state content should be backup and restored with the rest of the data. 

So, this issue is for:

• Adding support to backup/restore to the xAPI states implemented in core_xapi.
• Calling these methods from mod_h5pactivity to guarantee the xAPI states information is also backup/restored.