Ignore EXIF data in FormatMetadata::fetchExtendedMetadata()

fetchExtendedMetadata() gets extra data such as file templates
via the GetExtendedMetadata hook; keys which are absent in the
extended data default to their EXIF value.

This turned out to be a bad idea as EXIF data is too messy in
practice with various devices putting e.g. manufacturer name for
author name; completely ignoring EXIF data in the extmetadata API
is still the better option.

Bug: T73719
Change-Id: I77b1df67d08097504b3b1026c584063488e03b6e