Set default file format for existing module installs to XLIFF

Lately, we added support for XML file format in our translation provider. But we wouldn't like to unexpectedly change file format for existing clients. That's why it's a good idea to set it on hook_update_N.

Problem:
With new file format option in UI existing clients might automatically be switched to new file format.

Resolution:
Set this new option to XLF value during module upgrade on hook_update_N