FormatMetadata::fetchExtendedMetadata: Ignore multiple EXIF/XMP values

Ignore all but the first value of multivalued metadata fields. Before
this, the function returned an array of values for such fields,
causing various errors.

Bug: 72652
Change-Id: I0fd22b78e4938eecc506d870e352b3b196ee3ace
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/973
Merge "FormatMetadata::fetchExtendedMetadata: Ignore multiple EXIF/XMP values"