libjpegrecoverymap: XMP updates.

Actually write out MinContentBoost in the XMP. Also move XMP for
RecoveryMap:* elements into the secondary image Item, to match updates
to the ghdr spec. Update XMP test for min content boost.

Bug: 264715926
Test: tests pass
Change-Id: I8e32755ef3852511ee2e89949fcb7a3cd61d0507