Make DjVu metadata be stored as serialized PHP array.

Previously metadata was stored as a string of XML. Some of the
code in File expects the metadata to be an array serialized using
PHP's serialization format. In particular, this fixes the api
query=imageinfo module, and by extension instantCommons.

This supersedes Icaa16eeb

Bug: 37764
Change-Id: I5c1d2d2434f70b57137837bade797d4133c47b70
Merge "Make DjVu metadata be stored as serialized PHP array."