As we switch ParserCache to JSON, we can no longer serialized
class instances in extension data. PageImages was writing the
full set of properies it received from Parser into it's extension
data, some of which are sometimes class instances. Instead, only
write the nessessary subset of data into extension-data.
This change is completely forward and backward compatible.
Since before this change we were already writing the same array,
but with many additional unused properties.
Bug: T266251
Change-Id: Ieb4a139465159611e6b3a99c4b68c3c174b1944f
